I have made several pairs of gaiters for mountaineering boots: it is fairly easy to do. You need to decide whether to put a zipper in so the gaiter can be put on after the boot is laced up, this needs to be a separating zipper and should be fairly robust to withstand the dirt in its working environment. The rest of the job Is to decide how to shape the fabric. I cut waterproof pack nylon into strips about 10 inches high and roughly 18" long and then using trial and error tapered the ends to make a close fit at the boot top and around your lower leg. You need to leave a seam allowance and enough fabric for hemming, and for the zipper installation if chosen. I used some scrap fabric I had to make the pattern and then sewed the gaiters. You will want to put a light elastic in the top and need to put grommets on the sides for the tie cord that passes under the arch of the boot.

Personally, I didn't feel that the results were worth the effort when one can purchase good ready-made gaiters quite reasonably from sources such as REI. Of course making a light pair from Cuben or silnylon would be different. I haven't seen such listed for sale--but I haven't looked either. You can get good gaiters for trail running shoes from "Dirty Girl".