Squashing git commits into a single commit for a github pull request

There's lots of advice about how to git rebase. But unfortunately almost none of these address the case where you have a pull request on a repo with multiple merges to master. In this case you'll have lots of other people's commits in your history and you can't just do:

git rebase -i HEAD~3

Thankfully github documented the solution. 99% of the time what I want to do is squash all of my commits on a branch relative to the repo master branch. It's super easy once you know:

git rebase -i master