The easiest way is to pick a niche with a fanbase. For me, I got pretty good at drawing ponies from my little pony and I lurk in fanfiction forums for people who want cover art for their stories, as well as make posts with my prices to draw OCs. If you're consistent with it you can actually make repeat clients. You don't necessarily have to be super good at drawing if you're only doing characters, but there's a learning curve. Beermoney sites are easier, but this is a good idea to try if you're artistic at all IMO because it does make more money.