When possible, avoid syncing with the upstream using a branch and PR on the forked repository. Instead, merge directly into the forked main.
If a PR is absolutely necessary, use the following steps after checking out your branch:

   git checkout -b your-branch-for-syncing
   git pull --squash --no-commit upstream main
   git commit -m '<your message without GitHub references>'
   git push --set-upstream origin your-branch-for-syncing