Contribute to the FOSS projects that you use personally, here you know best why it's important, how it works, and what can be improved.

Start by reviewing the pull requests of other contributors to be useful instantly and learn your way around the code base. Then find an open issues that's in your interest and skill range, propose a solution, incorporate feedback, and deliver a pull request.

Do that for a while, and if you're actually useful, then someone is likely to toss you some sats.