How do I create and/or send a pull request to another repository hosted on GitHub?
I wrote a bash program that does all the work of setting up a PR branch for you. It performs forking if needed, syncing with the upstream, setting up upstream remote, etc. and you just need to commit your modifications, push and submit a PR.
Here is how you run it:
github-make-pr-branch ssh your-github-username orig_repo_user orig_repo_name new-feature
You will find the program here and its repository also includes a step-by-step guide to performing the same process manually if you'd like to understand how it works, and also extra information on how to keep your feature branch up-to-date with the upstream master and other useful tidbits.