I'm very interested in this and am trying to figure out how to do just a basic affiliate site setup.
If you write convertible copy, you can find an existing affiliate chain and try to use that to leverage your advantage. Otherwise, I recommend trying something that's out there, realize you won't be able to compete in your first niche (too much competition), then use that to learn.
Your first ROI might be 25%. Try again, find another niche, until you find the right one. This is classic explore/exploit game theory (every well publicized channel, e.g. the "Bingo Card Creator" of affiliate world) will be Red Queen'd to death.
note - not "great" copy, but copy that converts! (Or maybe you make landers that convert like crazy, or you have enough money to buy a ton of traffic and conversion optimize like crazy)
After that, I realized it was all about the basics + a niche. you need to know the fundamentals of "buy traffic, convert traffic to business goal, collect money for successful business outcome", but that was table stakes.
Once you know the mechanics of the game, it's up to you to niche/exploit. You're not going to make money doing what everyone else does, so if you're asking for advice instead of trying your own thing, you're already losing.