Note: if there’s a conflicting commit in the history of your master branch, you can destroy your branch and replace it with a fresh copy using the command
git checkout -B master upstream/master.
To make sure your commit goes in at the top of everything else on the upstream repo, rebase:
git rebase upstream/master
If there are conflicts, open the file and look for the diff markers, resolve, and continue.
For basic tips about using Pull Requests, see Creating a Pull Request
See also Pull Request Advanced Tips & Tricks
See also Approving Pull Requests