A bit of a different thread, but I looked into it where I live but it wasn't worth it
😛
Basically you just need to learn to processes of replacing screens, batteries, antenna's, LCD, Glass, home screen, other buttons etc.
You can learn this off of here:
https://www.ifixit.com/Guide
Just look at all the models, note that you will get the best business from repairing as many models as possible, as well as starting small and being extremely good at it.
If you want to do this, get some spare iPhones and take them apart slowly just to get the feel of doing so, and make sure you don't damage it, as you want the best reputation as possible.