Usually hands are good enough, but for more precision, I use dark red construction paper. Sometimes, it's just a simple hole, when it's more complicated, I place a box on the easel, project the image, put the paper on top, and trace the outline of the burn area with a pen. The cutout is made/used about 6" above the paper meaning the burn edges will be soft but fairly accurate. The use red paper because it doesn't bounce so much light around and so I can see the image while burning [I don't need to peek underneath].

-Tim