git-merge(1) prior to a PR in in the github.com/freebsd area

From: Graham Perrin <grahamperrin_at_gmail.com>
Date: Sun, 27 Aug 2023 05:30:12 UTC
I have a checkout of 
<https://github.com/grahamperrin/freebsd-src/pull/3>, and need to squash 
before I create a PR at <https://github.com/freebsd/freebsd-src/pulls>, 
but I can't make sense of git-merge(1).

<https://git-scm.com/docs/git-merge>

Can anyone steer me?

TIA