Check and apply the patch:

git apply --stat fix_empty_poster.patch
git apply --check fix_empty_poster.patch
git am --signoff < fix_empty_poster.patch

git format-patch -1

git format-patch gitlab/master --stdout

git checkout -b temp-squash master
git merge --squash some-name-identifying-my-work
git commit // ... remove the "Squashed commit of the following:" leader

git format-patch -M -s master

This will generate patch files in the outgoing subdirectory, automatically adding the Signed-off-by line.

git format-patch -M -n -s -o outgoing origin/master

git send-email --to your@email.address 0001-....