This is seriously the easiest and cheapest stamp to make. Especially if you want a simple shape. I only had to buy one supply, and the rest I had laying around the house.
craft foam sheet
xacto knife or scissors
paint - acrylic or fabric (not pictured)
The craft foam I used is like THIS but I used a thicker sheet. You can find it at any craft store in the kid craft section. First, cut out a triangle with scissors. But I would use an xacto knife for a more complicated shape. Add glue to the back.
Second, cut a rectangle from your cardboard. I got my cardboard from the flap of an old shipping box. No need to buy cardboard... I'm sure you have something laying around. Attach your foam shape to one side of the cardboard. Let it dry.
One side of my cardboard was white, so I decided to draw the stamp shape and I tried to make it cute by writing the name too. No need to do this, but it might be helpful if you are making a lot of stamps.
I used white acrylic paint for my black fabric (that's what I had lying around), but fabric paint works great too. I really don't mind using acrylic paint on fabric... I did it with my babie's onesies and they lasted just fine in the wash. So, just put some paint on a paper plate, dab your stamp and rub around in the paint to get it fully covered. Test on a scrap piece first, then take it away!
I used my stamp on fabric, but they are great for kids projects or paper crafts. Just about anything. I might even buy those pre-cut foam shapes you find in the same kid craft section at the store, and try to use those as stamps!