I don't know how intricate your butterflies are but if they were just simple shapes, I think I would use a used dryer sheet or something similar and stitch the outline of the butterfly. I would clip the curves and make a slit in the fabric softener sheet and turn the butterfly right side out. I would press it with the iron until I got it the way I wanted and hand applique it one. I'm sure you'll get lots of suggestions. Good luck!