Once you've included all the information you'd like to share and communicated with your friend about their life, you can finish the letter. Write a few sentences that reference your friendship and future correspondence.  For example, if you've been away, finish with, "It’s been a lot of fun, but it’d be even more fun if you were here. I’m looking forward to seeing you once I get home!” If you and your friend have been having a disagreement, write something like, “I know we’re in a tough patch now, but I want you to know that I'm grateful we’re working through this.” Choose a friendly sign-off followed by a comma. Then sign or type your name below the sign-off. For the most personal closing, write your signature instead of printing or typing it. Consider using any of these sign-offs:  Yours truly, With love, Hugs and kisses, Fondly, Take care, Cheers, Once you finish your letter, take a little break and then read through it, looking for grammatical or spelling errors. If you’re short on time, you might start by typing the letter out into a text document and then running a spellcheck on it. You’ll also want to check your letter to ensure that everything you say makes sense. Remember that your tone of voice can be difficult to translate via writing, so be careful that what you say is clear and won't be misinterpreted. List your friend's first and last name in the center of the envelope. Write their house number and street address on the line below it. Then write the city, state, and zip code on the line under it. Include all of your information in the same format in the upper left corner. If your friend is living in another country, be sure to include the name of the country in the address. Check how much postage is required in your country to mail a letter. Place the postage on the envelope's upper right corner. Lick or tape the envelope shut and drop the letter at the post office.  You may be able to leave the letter in your mail box. This usually means that you'll need to raise the small red flag on the side...

TL;DR in Vietnamese:
Hướng đến phần kết thư. Viết câu kết. Đọc lại thư. Ghi địa chỉ của bạn và người bạn lên bì thư. Dán tem bưu phí thích hợp lên bì thư và gửi thư.