How you specifically contribute is going to change per project. Most mature and popular OSS projects are going to have a Contributing.md file that would walk you through their process and what they expect from a contributor before attempting to make a PR. React Contributing.md.
For my project, Express Admin Area, you'll notice there is no Contributing.md and that's because I don't care. If someone does something blatantly wrong then sure I'll correct them or ask for a fix but it's pretty laid back.
Now if you're wondering about how to get your code into their code base then I would look over How to contribute but the idea is pretty much
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