Here I've used frisket film on Yupo and cut out simple shapes with an Exacto knife before applying my ink. For those not familiar with frisket film, it's a lot like transparent contact paper with a low tack adhesive. Most art suppliers carry it.
I've wanted to try this for some time and finally got around to it recently. It's fun to have the edges of the film corral the ink and to feel free to keep throwing color. It's almost impossible to make a mistake with this technique. These are a few of my results. I think they could handle some more embellishment, possibly some jelly roll pen or marker OR I may just leave these as is and give them a coat of gel gloss medium. I don't dare spray Krylon varnish in the house in winter.