How is the process of stretching the layout on the site?

Question

HTML

Good afternoon! the question is - what does the process of pulling the layout on an already working site consist of? Well, it turns out that they made a new adaptive layout and it needs to be implemented on the site, there are very few new functions - 2-3.
What should a programmer do, provided that he was given a ready-made layout and a working site with such a task? Pages - main + many internal.

Answer 1

If already working, then:

1. Copy the latest version for yourself and work with it locally first, so as not to screw up the working version
   
2. Further, change the unique templates in accordance with the new version, while NOT touching the old functionality, because. this should be done according to the principle "We supplement the functionality, but do not redo it", because it's like everywhere else, everything that was supposed to work as it worked, with rare exceptions. And also to minimize the changes made to the database, because. Ultimately, it is desirable to DO NOT REUPLOAD the database , but supplement it according to a short list of changes necessary.
   
3. Test it all