workflow for managing ports patches against /usr/ports in poudriere

Hi,

On IRC I've been advised to use overlays for this.
So let's say there's a port and the patches for it aren't in the tree.
I download a fresh ports tree, null mount it and make that my overlay tree.
Then strip out all the ports I don't want to patch, then patch the port in the
overlay.

Then

poudriere bulk -j builder -p /usr/ports -O overlay category/patched-port

is this correct? Or is the patch just put somewhere in the overlay tree?
Or is there some documentation explaining exactly what to do?
I have around five or so ports needing patches and the patches aren't in the
official tree yet.

The problem the above seeks to address is that, if /usr/ports is directly patched,
when the time comes to update the tree, git pull --ff-only complains.
Unless it's stashed then updated then un-stashed.
--