Tommy Griffith has a pretty thorough case study for how he created ClickMinded as a WordPress-based course / membership website, which in 2014 crossed into six figure revenue as a side project.
It's worth reading in general, but I really love his closing notes:
I think my advice for starting a new side project can be summarized in two points:
- Be passionate about what you’re working on
- Don’t stop wiggling
There are a ton of great ideas out there. There are a ton of profitable ideas out there. There are not a ton of ideas that you will personally be passionate about. What I’ve come to discover, is that when getting started, the size of the market is much less important than the size of your own personal interest in the market.