I've done ths with some success.
Scan the decal sheet, then print it on white paper.
take a piece of glass and stick down some masking tape, whatever you use.
cut out the decal off the white paper, leaving some overlap.
place it on the masking tape and use scotch tape to seal it in place.
Use an X-acto knife and cut your decal mask out, and carefully peel it off the glass.
the white paper decal may, or may not stick to the masking tape after you cut it out, but no matter.
The new cut mask on the lower tape is what you want.
Stick it in place and spray.