I'll start off by saying that I am no expert in this subject. However, I did learn just enough to put together what will become the most influential tutorial for modding and working with GTA:V interiors. The process is long. It's daunting and can easily be one of the most satisfying things you can do with GTA:V.

Without further ado here is that tutorial.
Here is also a larger document to go along with it.

GTA:V Mapping Notes
Digestible GTA:V Mapping Document Because I don’t like playing 52 card pickup on the internet. Thrown together by Stuyk I just don’t like wasting my time looking for information that should have been constructed together in a single digestible document in the first place. Thanks to those of you...