Hey, if you want to try to fight the "imposter syndrome", just go to a repository and find an issue you think you can solve. Then fork the repo, fix the problem and send a pull request (+mark a collaborator, they often don't see pull requests^^). They will take a look at your patch and they will tell you if it's ok or if you have to do anything more or make further changes to comply to their standards. I am pretty sure that your work will be welcomed, as finding people who submit PRs are quite rare compared to people who use the library or just complain about the bugs :)
If you'd like to have a kind of test run, you are welcome to come over to my little org on GitHub and implement a little something into one of the smaller projects. I will tell you what I think is good and how you might improve. You might want to take a look at this, that or that issue, which are basically just a thing of minutes to implement, once you understand the library.