You only need to perform these actions once on your local copy of your fork.
git remote add upstream https://github.com/mybb/mybb.git
Run these commands to create a branch fix-1234
with a starting point of the last commit from the mybb/mybb repo.
git fetch upstream feature
git checkout -b fix-1234 upstream/feature
Perform your changes then
git push origin fix-1234:fix-1234
Ensure you are on the correct branch e.g. git checkout fix-1234
git fetch upstream feature
git merge -ff upstream/feature
You will then need to manually resolve any merge conflicts then create a commit and push to your fork.