If you don't want the traditional garter tie look, you can also make "notched, flat" flashes from ribbon or belting products found at fabric stores. I recently made some red ones from cotton belting. Elastic garters are easily sewn up -- I used plastic snap fasteners, IIRC they are called "parachute buckles" or you can fasten them with Velcro.

There is also a post on this forum from Barb T regarding flashes from tartan, using a product called Steam-a-Lite 2 (again IIRC) if you have tartan scraps or can find green fabric in the color you need. But the ribbon versions look fine and are a lot less time/cost/work.

Good luck with this and congratulations on the big event!