I want to make my classic apps lightning ready, what are the things i have to consider, like i have to think of updating my VF pages using SDLS. what else do i need to think of ?


Being Lightning Ready has nothing little to do with using SLDS, at least not directly. The quickest way to learn about what needs to be done is to use the Lightning Readiness tool found in Setup > Lightning Experience (Classic Only); one of the menu items is:

Check Your Lightning Experience Readiness

We can evaluate some of your features and customizations to help you determine which users are ready to migrate to Lightning Experience. After you enter your username and password, we email a readiness report to you. Tell me more

This will involve things like checking URL buttons and links, JavaScript buttons and links, hyperlink and image formula fields, and so on. Not all items need/require updates, but the LER is aggressive in recommending changes, preferring false positives to false negatives. You'll need to evaluate each mentioned item by itself, which will result in one of three actions: discard the item, perhaps because it's now obsolete, update the item to be LEX-compatible, or doing nothing because it already works in LEX.

Also, you can't just generally slap SLDS on to a Visualforce page; even using Lightning for Visualforce has some limitations, such as some objects not being styled correctly. The only way to know for sure is to test your pages out in Lightning. Remember to test on a phone and tablet as well, as they have different experiences than desktop users.

Many of the buttons and links that are not compatible will need to be written as Lightning components, and then added as Actions on the object. Many of your Visualforce pages would be better off written as Lightning Components and then served up using Lightning Out to service non-Lightning clients. This provides a forward upgrade path for your clients.

Also, any JavaScript you have in your VF pages, either directly or by way of <script> or <apex:includeScript> will need to be tested for compatibility. For example, you should not use window.location whilst in LEX or SF1 (use force:navigateToURL instead).

So, your checklist should include checking your Lightning Experience Readiness report and evaluating any outstanding items, testing all of your VF pages in LEX on both desktop and mobile devices, and adding SLDS to your VF pages or converting to Lightning Components.
