You carve it up - kind of like whittling. Seriously, I remember doing this as a kid. Because of the tecture, you can get a really neat print. - try for instance, making a heart: make a paper stencil of heart. cut the top of a potato so you have a flat surface. Using the paper stencil, trace it on the potato. With a small paring knife, cut away the perimeter of the shape so the shape is about 1/2 inch over the rest of potato. Voila, primitive stamp. Of course, you've only got so long to use it before it rots, but it's way cool if you're wanting to get that sponge-pressed look. Great fun for the kids in your life too.