I built a Slack bot called Standup Jack (https://standupjack.com). It's a ton of work upfront but if you launch early, keep an open dialog with your users, iterate on their feedback and keep your costs low, it's a nice way to earn some side income each month.
Udemy is a pretty easy way to start since they do the marketing for you (for a pretty big cut).
Would love to hear!
1) Direct marketing - Udemy drives a ton of traffic to their website, so people just naturally see your course(s) and buy it.
2) Affiliate marketing - Udemy advertises your course(s) via things like Google Adwords. You make a very measly cut on these sales.
3) Personal marketing - You can promote your course through your own channels / reputation. You make like 98% of the sale when people purchase it this way.
Without doing your own marketing, you can easily earn decent side income though.