I'm going to learn about making money via subscription payments. I've launched a few apps, some that I'm very proud of, but nothing that attempts to make money

I'm going to learn about making money via subscription payments. I've launched a few apps, some that I'm very proud of, but nothing that attempts to make money in the most straightforward way: charging for use. Consequently I've never felt like I really understood this simple thing, or how easy it would be to deploy it on a new or existing app. More broadly, my goal is to learn about bringing a business to life. Having quit my day job and switched to consulting in the last year, I'd really like to start exploring new opportunities that could make money while I sleep.