Launch an import. If there's nothing to import gbp pq will just create an empty branch and switch your working copy to it:

  gbp pq import

Create your first patch: edit files, test, commit your changes using git commit

To generate the new Quilt patch set use

  gbp pq export --commit

This will switch you back to your debian-branch branch, generate the patches and commit them right away to this branch.

Skip the --commit if you don't want to commit right away. If you want to pick the changelog message from the patch see /usr/share/doc/git-buildpackage/examples/gbp-add-patch.