I learned it in my kinder garden when I was a little kid. We used it to decorate cards and and drawings. It makes good prints on clothes too :)
Take a washed potato and cut a 1/3 of it. Dry it with a napkin.
Embellish surface with a razor so it would be smooth as a mirror.
Draw a desired form on the surfase.
Cut around it with a sharp knife and careful remove the part around your drawing. Then dry it with napkin again.
Now you have your stamp. Dip it into paint. They must be not to liquid.
Voila! Here's the result. Use it as you want.
Dip it into acrylic paint and decorate your jacket, leather skirt, purse or whatever you want.
The potato will rot eventually. To make it stay fresh longer keep it in water with a little bit of lemon juice. But it cost nothing, so you can make another stamp whenever you want :)