user10210: I started coaching a child (11 yo) part-time.
Has to be said that I am passionate about chess, and I am myself 1992 ELO and am new to coaching, but I like sharing and explaining.
At first I was helping him to prepare before the tournaments and we went through different ideas specially preparation for his opponents. I have noticed this was too much and it's going too broad... 
To be also said that his level is good, he understand main strategies, ideas and he's pretty good at tactics as well.
I have proposed him to create an opening repertoire. I was thinking at something very narrow but solid and expandable. I know what type of positions he enjoys playing, so opening will be easy to select, the only problem is I don't know how to start the process of teaching him this. 
(Myself I never had a coach so I have learned alone mostly by practicing, and I don't really have a great repertoire, so I cannot use my repertoire creation as experience to help him)
Do I need some tools, some special ways of starting with specific lines, ideas? 
How does a professional coach teaches a child a chess repertoire, step by step?
Thanks

user18809: Despite there already being some great answers, let me emphasize a few things: Take into consideration that we are not trying to build a repertoire from scratch. Your student probably knows already a few things about certain openings. Use that knowledge and expand on it.
You should find a repertoire that powers his strenghts and is comfortable to play for him, but also one that teaches him "good chess" in the long term. I'd thus prefer lines that emphasize relevant strategical themes by trying to use them into building an advantage (for example, if you chose the French defence, go for dynamic Winaver lines rather than ...dxe4 approaches!)
Since you are not much stronger than your student, make sure you do understand the openings you'll teach him. If necessary, talk to a master deserving of your trust for specific lessons.
As you mentioned preparations: they are often important. So if you are going to spend time building a repertoire, make sure it's not one that becomes an easy victim for those preparations (otherwise it would have been better not to have a repertoire at all) 
As for your own preparations, please don't fall victim to the pursuit of a short-term success. Every chess player must develop his skills to thrive in unknown territory! Indeed one of the mistakes I often see in young players is that they have no clue what to do when they are "out of theory" earlier than his opponent

user14079: It is well worth reading Mikhail Shereshevsky's advice for trainers building a chess repertoire for their pupils.
I am afraid this quite a long quote, but this section can be found online free of charge here(pdf), and I encourage readers who want to learn more to purchase Shereshevsky's book or e-book, 
The Shereshevsky Method to Improve in Chess.

When one begins work with a young first-category player or candidate master, of between 10-14 years of age, one is dealing with someone whose style has not yet been developed. He has a great deal still to learn. As a rule, within a few lessons, one needs to carry out a chess diagnosis. In my day, this was definitely the case. Nowadays, when I get to the Russian junior championship, it seems to me that things have not changed for the better. Since you are often unclear about the youngster’s style or preferences, trying to guess whether he will be comfortable with this or
  that opening is pointless and just a waste of time.
  The opening repertoire of such a young player needs to fulfil the following
  conditions:
  1) It should be solid, with a firm positional basis; active, but not too
  aggressive.
  2) It should not include flawed or unsound openings, which, as the
  player advances and starts meeting stronger opponents, will sooner or
  later be more or less refuted. They lead to significantly inferior positions
  and give the opponent odds at the start of the game. I have in mind such
  openings as, against 1.e4, Alekhine’s Defence, the Scandinavian, Philidor,
  etc. Although there is a temptation to use such openings to achieve shortterm
  successes against weaker opponents, sooner or later the player ends
  up ‘bankrupt’ and has to master a new opening from scratch.
  3) The opening should not involve avoiding a whole raft of opening
  ‘tabiyas’. Thus, I would not advise a young player to open 1.♘f3, 2.g3, 3.♗g2,
  4.0-0, 5.d3, 6.♘bd2, 7.e4 for his entire life, or 1.♘f3, 2.c4, 3.g3, 4.♗g2, 5.0-0.
  His strategic thinking will be too limited and will not develop widely
  enough. His games will lead to an excessively narrow range of positions, in
  terms of strategic content. If you are going to play closed openings, then it
  is better to play 1.d4, 2.c4 and preferably 3.♘c3, not 3.♘f3.
  4) Most important of all: the repertoire should be such that one does not
  need to pay any great attention to it more than once or twice a year, other
  than via analysing the games played.
  Once your students have become masters or grandmasters, their chess
  style and tastes will be fully formed and then they can completely
  overhaul their opening repertoires, if they so wish. But whether you teach
  them to play the Spanish, Sicilian, Caro-Kann or French is of no great
  significance. The important thing is to establish a sound positional basis.
  First, the player should learn to play the game in all its many aspects, and
  approach the opening stage rationally, not primitively.

Great principles, which may not be that easy to fully implement, but are worth been used as guidelines at the very least.

user14079: I know what type of positions he enjoys playing, so opening will be easy to select.
In that case the hardest part is already behind you. Let me suggest the following strategy: Copy the repertoire of a grandmaster.
Pick a high-level player would style you think would fit you pupil. The correct pick should be a principled player, who uses regularly the same openings, and who plays quite a lot. For instance, in my practice as a teacher I have copied parts of the repertories of GMs Leko, Gelfand, Kozul, Tomashevsky... Avoid guys like Ivanchuk, Carlsen (at least pre-2015 Carlsen) or Rapport who just try everything !
Then, if you have chosen a Najdorf player (say, MVL) as a model for Black, you can then answer most choices just by looking at MVL games: What to do against 6.Bg5 in the Najdorf ? Well, MVL plays 6...e6, let's study a couple of his games. What do you against the closed Sicilian ? Let's see how MVL copes with it, etc.
You don't have to copy the whole repertory of course, GMs often have several pet variations in every openings. MVL also plays 1...e5, but if you decided to copy his Sicilian repertory it is already well enough, an 11-years old doesn't need as broad a repertory as a top-level player. Nor as deep, of course, don't bother about novelties after move 15.
What are the advantages ? 

Your repertory is super reliable. If a top player trusts it, it will not be refuted at club level. Moreover, if one line is in theoretical trouble after some novelty, you can just see how your model GM copes with it: if he deviates earlier, just follow his lead.
Your repertory will be coherent. The GMs has thought a lot about transpositions and tricky move orders (which are clearly not what you want to lose time on while teaching). You can trust him for his answer to 1.Nf3 will not lead you by transposition into a 1.d4 opening you know nothing about.
While building his repertory, you pupil will have to study high-level games. They are the best examples for him to make progress not only in his opening play, but also in the middlegame and in the endgame.
If your chosen model is active enough, especially if he plays a lot online, you can expect him to have faced most variations and sub-variations, and you can easily collect his games so that your repertory will be quite complete. If really there is a line no-one has ever played against him, you can always switch to another player or to a reference book.
If you are not sure whom to pick, give a selection of games from 3 or 4 GMs for your pupil to replay. There will probably be one he likes more than the others...


user19877: A repertoire is not built overnight, and for most players, having a deep theoretical knowledge is not required. They simply need to understand opening pawn structures.
That said, if this child is showing a great deal of aptitude, it may be worth adding this to your study regime earlier than I would normally recommend. I have seen two prodigies up close, and I will share what I have seen: I have known GM Josh Friedel since he was 6 or 7, and his first coach, who was the first player that I REALLY wanted to beat, has been a close friend for 40 years, so I know that he taught Josh openings even at a very young age.
My second encounter with a prodigy is the amazing 8-year-old Ryan Sun, who is now already rated above 2000 (and plays stronger than that). I faced him in my first game back after 16 years, and again, just this past weekend. I went 0-2, but in our game this weekend, it was especially clear that he was well-versed in the opening. So, for certain kids, who are gaining strength at an incredible pace, they do need a repertoire early since they will soon be facing players, who are booked up.
Unless you are an expert at opening play, you need to do one of two things at a certain point: Pass him off to a stronger coach. In Josh's case, he "graduated" from my friend to a GM coach, I think at around age 11. The other option is to buy repertoire books, that were written by grandmasters.
For kids, who calculate like little computers, you probably want books that focus on aggressive repertoires, typically 1.e4 as white, the Sicilian Defense as black versus 1.e4, and then a GM-suggested repertoire versus 1.d4. There are a lot of such repertoire books out there.
I hope this helps.
Below, the original poster asks about how to prepare to present the material, and how to prepare to present it, and I added a comment. I will that here too.
Usually, those books are pretty complete repertoires, and you need to go through a good portion of the book to make sure you cover everything. So, take each chapter, and they do not have to be in order (for example, if the first chapter is oddball sidelines, that is not the place to start), and read it yourself, and get a feel for the plans and ideas; and then start presenting them chapter by chapter until you are done with the book. He will not have to memorize everything, but this way, he will be exposed to all of it. Just keep building on that.

10210: <p>I started coaching a child (11 yo) part-time.<br>
Has to be said that I am passionate about chess, and I am myself 1992 ELO and am new to coaching, but I like sharing and explaining.</p>

<p>At first I was helping him to prepare before the tournaments and we went through different ideas specially preparation for his opponents. I have noticed this was too much and it's going too broad... <br>
To be also said that his level is good, he understand main strategies, ideas and he's pretty good at tactics as well.</p>

<p>I have proposed him to create an opening repertoire.<br> I was thinking at something very narrow but solid and expandable. I know what type of positions he enjoys playing, so opening will be easy to select, the only <strong>problem</strong> is I don't know how to start the process of teaching him this. <br>
(Myself I never had a coach so I have learned alone mostly by practicing, and I don't really have a great repertoire, so I cannot use my repertoire creation as experience to help him)</p>

<p>Do I need some tools, some special ways of starting with specific lines, ideas? <br>
How does a professional coach teaches a child a chess repertoire, step by step?</p>

<p>Thanks</p>
 18809: <p>Despite there already being some great answers, let me emphasize a few things: Take into consideration that we are not trying to build a repertoire from scratch. Your student probably knows already a few things about certain openings. Use that knowledge and expand on it.</p>

<p>You should find a repertoire that powers his strenghts and is comfortable to play for him, but also one that teaches him "good chess" in the long term. I'd thus prefer lines that emphasize relevant strategical themes by trying to use them into building an advantage (for example, if you chose the French defence, go for dynamic Winaver lines rather than ...dxe4 approaches!)</p>

<p>Since you are not much stronger than your student, make sure you do understand the openings you'll teach him. If necessary, talk to a master deserving of your trust for specific lessons.</p>

<p>As you mentioned preparations: they are often important. So if you are going to spend time building a repertoire, make sure it's not one that becomes an easy victim for those preparations (otherwise it would have been better not to have a repertoire at all) </p>

<p>As for your own preparations, please don't fall victim to the pursuit of a short-term success. Every chess player must develop his skills to thrive in unknown territory! Indeed one of the mistakes I often see in young players is that they have no clue what to do when they are "out of theory" earlier than his opponent</p>
 14079: <p>It is well worth reading Mikhail Shereshevsky's advice for trainers building a chess repertoire for their pupils.</p>

<p>I am afraid this quite a long quote, but this section can be found online free of charge <a href="https://www.newinchess.com/media/wysiwyg/product_pdf/9056.pdf" rel="nofollow noreferrer">here</a>(pdf), and I encourage readers who want to learn more to <a href="https://www.newinchess.com/the-shereshevsky-method-to-improve-in-chess?ct=t%281245B%20eBookoftheW%20-%20The%20Power%20of%20Pawns%20dol_COPY_01%29&amp;mc_cid=7cb45925d3&amp;mc_eid=%5BUNIQID%5D" rel="nofollow noreferrer">purchase</a> Shereshevsky's book or e-book, 
<em>The Shereshevsky Method to Improve in Chess</em>.</p>

<blockquote>
  <p>When one begins work with a young first-category player or candidate master, of between 10-14 years of age, one is dealing with someone whose style has not yet been developed. He has a great deal still to learn. As a rule, within a few lessons, one needs to carry out a chess diagnosis. In my day, this was definitely the case. Nowadays, when I get to the Russian junior championship, it seems to me that things have not changed for the better. Since you are often unclear about the youngster’s style or preferences, trying to guess whether he will be comfortable with this or
  that opening is pointless and just a waste of time.<br><br>
  The opening repertoire of such a young player needs to fulfil the following
  conditions:<br><br>
  1) It should be solid, with a firm positional basis; active, but not too
  aggressive.<br><br>
  2) It should not include flawed or unsound openings, which, as the
  player advances and starts meeting stronger opponents, will sooner or
  later be more or less refuted. They lead to significantly inferior positions
  and give the opponent odds at the start of the game. I have in mind such
  openings as, against 1.e4, Alekhine’s Defence, the Scandinavian, Philidor,
  etc. Although there is a temptation to use such openings to achieve shortterm
  successes against weaker opponents, sooner or later the player ends
  up ‘bankrupt’ and has to master a new opening from scratch.<br><br>
  3) The opening should not involve avoiding a whole raft of opening
  ‘tabiyas’. Thus, I would not advise a young player to open 1.♘f3, 2.g3, 3.♗g2,
  4.0-0, 5.d3, 6.♘bd2, 7.e4 for his entire life, or 1.♘f3, 2.c4, 3.g3, 4.♗g2, 5.0-0.
  His strategic thinking will be too limited and will not develop widely
  enough. His games will lead to an excessively narrow range of positions, in
  terms of strategic content. If you are going to play closed openings, then it
  is better to play 1.d4, 2.c4 and preferably 3.♘c3, not 3.♘f3.<br><br>
  4) Most important of all: the repertoire should be such that one does not
  need to pay any great attention to it more than once or twice a year, other
  than via analysing the games played.<br><br>
  Once your students have become masters or grandmasters, their chess
  style and tastes will be fully formed and then they can completely
  overhaul their opening repertoires, if they so wish. But whether you teach
  them to play the Spanish, Sicilian, Caro-Kann or French is of no great
  significance. The important thing is to establish a sound positional basis.
  First, the player should learn to play the game in all its many aspects, and
  approach the opening stage rationally, not primitively.</p>
</blockquote>

<p>Great principles, which may not be that easy to fully implement, but are worth been used as guidelines at the very least.</p>
 14079: <p><em>I know what type of positions he enjoys playing, so opening will be easy to select.</em></p>

<p>In that case the hardest part is already behind you. Let me suggest the following strategy: <strong>Copy the repertoire of a grandmaster</strong>.</p>

<p>Pick a high-level player would style you think would fit you pupil. The correct pick should be a principled player, who uses regularly the same openings, and who plays quite a lot. For instance, in my practice as a teacher I have copied parts of the repertories of GMs Leko, Gelfand, Kozul, Tomashevsky... Avoid guys like Ivanchuk, Carlsen (at least pre-2015 Carlsen) or Rapport who just try everything !</p>

<p>Then, if you have chosen a Najdorf player (say, MVL) as a model for Black, you can then answer most choices just by looking at MVL games: What to do against 6.Bg5 in the Najdorf ? Well, MVL plays 6...e6, let's study a couple of his games. What do you against the closed Sicilian ? Let's see how MVL copes with it, etc.</p>

<p>You don't have to copy the whole repertory of course, GMs often have several pet variations in every openings. MVL also plays 1...e5, but if you decided to copy his Sicilian repertory it is already well enough, an 11-years old doesn't need as broad a repertory as a top-level player. Nor as deep, of course, don't bother about novelties after move 15.</p>

<p>What are the advantages ? </p>

<ol>
<li><p>Your repertory is super <strong>reliable</strong>. If a top player trusts it, it will not be refuted at club level. Moreover, if one line is in theoretical trouble after some novelty, you can just see how your model GM copes with it: if he deviates earlier, just follow his lead.</p></li>
<li><p>Your repertory will be <strong>coherent</strong>. The GMs has thought a lot about transpositions and tricky move orders (which are clearly not what you want to lose time on while teaching). You can trust him for his answer to 1.Nf3 will not lead you by transposition into a 1.d4 opening you know nothing about.</p></li>
<li><p>While building his repertory, you pupil will have to study high-level games. They are the best examples for him to make <strong>progress</strong> not only in his opening play, but also in the middlegame and in the endgame.</p></li>
<li><p>If your chosen model is active enough, especially if he plays a lot online, you can expect him to have faced most variations and sub-variations, and you can easily collect his games so that your repertory will be quite <strong>complete</strong>. If really there is a line no-one has ever played against him, you can always switch to another player or to a reference book.</p></li>
<li><p>If you are not sure whom to pick, give a selection of games from 3 or 4 GMs for your pupil to replay. There will probably be one he likes more than the others...</p></li>
</ol>
 19877: <p>A repertoire is not built overnight, and for most players, having a deep theoretical knowledge is not required. They simply need to understand opening pawn structures.</p>

<p>That said, if this child is showing a great deal of aptitude, it may be worth adding this to your study regime earlier than I would normally recommend. I have seen two prodigies up close, and I will share what I have seen: I have known GM Josh Friedel since he was 6 or 7, and his first coach, who was the first player that I REALLY wanted to beat, has been a close friend for 40 years, so I know that he taught Josh openings even at a very young age.</p>

<p>My second encounter with a prodigy is the amazing 8-year-old Ryan Sun, who is now already rated above 2000 (and plays stronger than that). I faced him in my first game back after 16 years, and again, just this past weekend. I went 0-2, but in our game this weekend, it was especially clear that he was well-versed in the opening. So, for certain kids, who are gaining strength at an incredible pace, they do need a repertoire early since they will soon be facing players, who are booked up.</p>

<p>Unless you are an expert at opening play, you need to do one of two things at a certain point: Pass him off to a stronger coach. In Josh's case, he "graduated" from my friend to a GM coach, I think at around age 11. The other option is to buy repertoire books, that were written by grandmasters.</p>

<p>For kids, who calculate like little computers, you probably want books that focus on aggressive repertoires, typically 1.e4 as white, the Sicilian Defense as black versus 1.e4, and then a GM-suggested repertoire versus 1.d4. There are a lot of such repertoire books out there.</p>

<p>I hope this helps.</p>

<p>Below, the original poster asks about how to prepare to present the material, and how to prepare to present it, and I added a comment. I will that here too.</p>

<p>Usually, those books are pretty complete repertoires, and you need to go through a good portion of the book to make sure you cover everything. So, take each chapter, and they do not have to be in order (for example, if the first chapter is oddball sidelines, that is not the place to start), and read it yourself, and get a feel for the plans and ideas; and then start presenting them chapter by chapter until you are done with the book. He will not have to memorize everything, but this way, he will be exposed to all of it. Just keep building on that.</p>
