Say someone has created a branch for you off of master, and they want to create Pull Requests (PRs) against it. Here’s how you do it.
Get on your fork (make sure sync’d and clean) and then do
git fetch remote branch git checkout branch
So for example
git fetch upstream imessage git checkout imessage
This grabs the upstream branch, pulls it local, and then when you checkout a branch of the same name, it automatically sets it to the upstream branch you just checked out.
Now you can create a new branch off this one, do your work, and merge it back by creating a PR via GHE.
git checkout -b foo`
You are now ready to work