- click around to find the author's repository URL
- git remote add it
- git fetch that remote
- copy the author's issue branch name from the pull request page
- git merge that
- run the automated tests, and check functionality
- then either git push or delete my own master branch and then check it out as a tracking branch from origin/master
That last step is probably suboptimal, but it doesn't happen often and I know how to do it that way. It illustrates my point, though: why doesn't github tell me how to do it?