Here is what you need (minus the butter knife and pen or marker) to make a rubber stamp:
First we had Zachary draw a picture (you can also have your child draw directly on the eraser). We went over it with pencil and placed the paper, face down, onto the back of the eraser. After rubbing, we darkened our picture on the eraser and cut around it with a butter knife. For finer details, we used the metal casing from a pencil (we were supposed to have removed just the eraser with the pliers, but our eraser was stubborn). If you need to make small holes, you can use a toothpick. More-detailed instructions can be found at Letterboxing North America.