1. Merge `debian/buster` into `debian/bullseye`. There will be a merge conflict in the changelog. You will need to accept the new changelog entry from debian/buster, but change the version from `+deb10` to `+deb11`.
1. commit
1. clean merge files with `git clean -f`
1. Cherry-pick any changes to the debian directory of debian/buster into debian/bullseye.
1. Make any other needed packing changes (but consider making them in debian/buster first).