Promoting casinos and referring gamblers is one of the best ways to make money especially if you referred whales or players who upgrade to VIP but the competition is very stiff, high rollers usually sign up to affiliates who offer them a percentage of their income but if you're good at it the best way is to first compile the most trusted casinos in the industry, don't think of just making money refer people to reputable casinos, so you'll not be blamed by gamblers for referring them to scam casinos.
I've seen review sites still promoting known scam casinos because of the high payout they are getting as a promoter.