Quick clarification by: "pick a substantial sub-project (not something trivial), start contributing to it", I mean:
_Implement_ a non-trivial module for the project in question , i.e., do much more than fix a few bugs.
Since you are junior (no actual development experience), it might not be realistic to hope for a remote position even in companies otherwise friendly to it. You need to make it a point to move for at least a few years: you'll want a company that can invest in you and give you a chance to prove yourself, before you can do more unorthodox things.
_Implement_ a non-trivial module for the project in question , i.e., do much more than fix a few bugs.
Since you are junior (no actual development experience), it might not be realistic to hope for a remote position even in companies otherwise friendly to it. You need to make it a point to move for at least a few years: you'll want a company that can invest in you and give you a chance to prove yourself, before you can do more unorthodox things.