Vinyl Stencil Window Painting.

[Gino]

Ohh..... that's another thing. I use paper frisket when doing this kinda stuff. You can get ripple or flat. Most people prefer ripple, but I always do the flat. It has the best cutting edge and keeps things about 99.9% crisp and doesn't have much peeling as you're describing, but still it should never be done completely dry.

Not many places carry it anymore. We have both 15" and 30" in stock.

 

[Jillbeans]

I hate Gerber's mask with the fire of a thousand suns. I prefer Oracal's, it is much more forgiving.
I'd pen plot or hand draw a big old paper pattern, tape it to whatever side of the window you're not painting, and use that as a guide. So much less expensive and much more authentic.

I'd worry about scratching the glass with an X-Acto and would never do that method. If you must use a mask, peel when wet, as others have suggested.
Love...Jill
PS
nice work.