I wrote up the basic steps below, but I have more step-by-step pictures and details on my blog.
14 packets of plain Knox gelatin (about 4 boxes with 4 packets in each box)
4 cups water
Jellyroll or sheet pan
Water-soluable printing ink
Leaves, flowers, or anything else with a fun texture or shape
Collect leaves or other objects with a neat texture or shape (the flatter, the better). Put some of the printing ink in a flat dish, roll the brayer in the ink, then apply the ink to the gelatin. Once it’s more or less even, place your leaves or other objects on top of the ink and press a sheet of paper on top, rubbing the paper thoroughly over the ink. Carefully peel the paper up and you have your first print.
Print to your heart’s content, experimenting with ink colors, objects, maybe even drawing something in the gelatin. If you want to change ink colors, you can wipe the gelatin carefully with a slightly damp cloth. The gelatin will also keep for at least a day or two on the counter.
Allow the prints to dry, frame, and say “Holy crap, I made that!"