One of the smartest purchases I've made for my costume and prop work is a vinyl cutter. I immediately put it to use making custom vinyl stickers, but it finds most of its use as a stencil creator. I'm often trying to create something identical to an existing piece, and doing it multiple times, so I want it to be consistent and speed up my paint process. I'm a freaking pro at masking these days but it's an onerous task. This saves my patience and my sanity!
One of the first stencils I made was for Sabine Wren's chest piece. I had bought one of the kid's costumes from Rubies to use as a template for armor plates so I scanned it and used my Silhouette Cameo software to convert it to a stencil. Now I can scale it up or down as needed and I have an identical stencil every time.
Don't have a vinyl cutter? Use cardboard or frisket film or paper or whatever else you have on hand!