Serial blogger Matt Stauffer wrote a good tutorial on how use forked repos.
I just recently joined a new open source project, and there were a few folks on the team who weren't familiar with how to contribute to an open source project by forking your own copy, so I wrote this up for the docs of that project. I figured I'd also share it here.
If you join a new open source project, it's very likely that you won't get direct access to push commits or branches up to the repository itself. So, instead, you'll fork the repo, make the changes on your version of the repo, and then "pull request" your changes back to the original.
Here are the steps to take.