Doing it on the keyboard could very easily work, but I have a laser-cut jig that holds twenty-six 1u keycaps, and has one open-ended spot for anything up to maybe 3u. I considered 3D-printing the jig, but 30 minutes on the laser made more sense than 5 hours on the printer.

This plastic didn't love my "Infusible ink" pens, so the legends are duller than the last time I did this, but the jig helped a lot alignment, as did adjusting my ambitions and expectations. Much less disappointing to land 1 off when you can pretend you wanted it there all along, and that is much easier to do with corner legends versus centered. :-)