Sometimes removing all the extraneous junk from a composition makes it easy to see. All it takes is a piece of cardboard and an Xacto knife.
Photographers and art directors in pre-Photoshop days would figure out how to crop a photo by setting it on a lightbox and covering up the edges with paper/cardboard/whatever. I was fascinated by it; a half-inch of concealment could turn a bad photo into a great image.
Why not apply the same technique to glass?
I do that when I’m working with flat part sheets I want to cut up and turn into pendants, or components in a larger work. Since glass STILL doesn’t come with an undo button (Bullseye, you need to get on that right away), it’s a fast way to see what you’re going to get before the cutter touches the glass. In the example above, I’m doing it with frit-sandwiched glass sheets.
It ain’t exactly rocketscience: Get a sheet of cardboard (or, if you want permanent template, masonite or wallboard). Draw the shapes you need, get an Xacto knife and cut them out.
All you have to do now is position your homemade template on the glass, and move it around until something strikes your fancy. Then outline the window with a Sharpie marker, and make your cuts. Voila.
This is also a nice way to quickly sketch shapes when you need a LOT of cutouts, although for that you may want to make your template in clear vinyl. It makes it easier to see where you’ve been.