There are lots of tutorials on how to make stamps from erasers (look at the Google search result for ‘eraser stamp’).
I want to present three of them:
Some hints & tips for eraser-carving
This site shows how beautiful those stamps can be and gives helpful hints.
Kneaded Eraser Stamps
Craft Chi always have ingenious ideas and tutorials and I like the kneaded eraser stamp because it is a technique to create stamps from everything with a textured surface.
Making and Using Stamps with Watercolor
I liked this one because I don’t stamp so often and so I don’t want to spent too much money on ink pads. Using watercolour makes it easy to use different colours nonetheless.
By the way: On the photo you see my first attempts on carving stamps from erasers and although they’re very simple I still like to use them from time to time.