I need to do this fairly frequently, and for some reason I can never remember the steps, so I'm writing it down!

This isn't news, it is lifted from the GitHub Configuring a remote for a fork and Syncing a fork pages.

$ git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git

$ git fetch upstream
$ git checkout master
$ git merge upstream/master

Make your edits, etc. Push (this will push to your fork), make pull-request. Next time you need to sync the orogin, start at the 'git fetch upstream' step.