Yes, that's pretty much it. Suckless.org provides instructions on applying patches to DWM, and the procedure is pretty simple. I'll add a few more details, though. I'm assuming that you're building dwm from source, since you'll need to do this in order to install patches.
Save the patch somewhere in your source directory, and run
git apply pertag.diff.
If the patch added any new files, run
git add <name of new file> to add them to the repository. If no new files were added, ignore that. Once you've added any new files, run
git commit -a to commit the changes.
Rebuild dwm. If you're building dwm as root, run
make clean install from the top folder of dwm's source tree (e.g., if you downloaded the source into
/build/dwm, run the command in that folder). It's the folder with the makefile. If you are building/installing dwm as a non-root user, just run
make from the source folder.
If you already have your system set up to use dwm, either through installing it as root or building it as a normal user and pointing to your user's copy, you shouldn't have to redo any configuration. Just restart dwm and the patch should be working.
A few more details. Technically, you don't need to commit any changes to the git repository, but I always do this because I maintain my copy of dwm in it's own repository, and pull in patches, developers' changes, etc. into it and merge them when necessary.
Some useful tips:
If you want to see what the patch does before applying it, download it and run
git apply --stat pertag.diff. This will show you what files the patch will change.
If you want to test out the patch, apply it using
git apply as I described above but don't commit the changes. Test out the changes (restart/rebuild dwm, use the new feature, etc.), then, if you want to roll back the changes, run
git apply -R pertag.diff.