The first step is to look at the patch, and make sure it isn't malicious. Everyone is vulnerable today; I would never produce malicious code writing documentation, so if you see this, please notify me and the security officer immediately.
Once you have examined the patch, the next step is to apply it. For FreeBSD reviews, use:
git arc patch -c D#####
For patches on GitHub, use:
gh pr checkout ####
I test manual by reading them side by side with the originals in
tmux(1), at MANWIDTH 59 and 80,
and using mandoc -Tlint, the compiler's linter.
When reading the rendered documentation, taste it.
Could it be clearer? More consistent with the rest of the manual?
Create a test directory or snapshot and test the examples.
Use apropos(1) and examine similar or related manuals.
Remember that manuals from an incredible array of operating systems and versions are available at man.freebsd.org.