I've done this a few times so lemme give you a quick rundown
You'd most likely be best off starting with making the individual patches, then testing the fit on the model, such as:
Keep in mind, however that retexturing on GTA V is shit, so you're most likely going to face some deformities & stretches on the model, these can be amended by just moving them around and fitting the patches elsewhere.
You can't really "find" ready-made templates, there's a few floating about in the community, but other than that they're all mostly sold on other communities for horrendous prices (i.e bikermods), I know of the Lost MC model being used a lot so that's going to be your best bet if you don't have many friends / people who are willing to share on here. but if you look hard enough, I'm sure you'll find something... eventually.
There's resources all over the internet, I'm not sure what you're asking in particular.