Another approach if you don't mind...
Quite a few years ago, I was in the same place you are. I knew I wanted to do my own upgrades but hadn't ever done one. I decided to have a sample of engines professionally upgraded: a brass steam locomotive with a large tender, a brass steam locomotive with a smaller tender and a twin motor Diesel. When I received them back from the installer, I took them apart to see how it was done. I now had something to copy. Copying is easier than figuring it out on your own.
I now do all of my own upgrades. I've done over 40 to date.