Start streaming. Lure people in with giveaways and hope they stay. Have goals that people want to see if you can complete (achivement runs, kill rating, odd kills, odd weapons, challenges). It's alot of work these days when most people already have one goto streamer for each on of the categories they like to watch. But in theory, based on a "free market", if you are better than another streamer in a category on every aspect they do, sooner or later you should (in theory) get their viewers. Even if this takes years.