First, you want to make a piece of duct tape fabric. See my very first post if you don't know how to do it.
For the perfect wallet size, you'll need 8 pieces of duct tape. I like to cut them all out and stick them to the edge of the table, just be careful they don't stick to each other or to itself. You'll want to cut them to be just a little bit longer than the length of the bill.
After making the fabric from these 8 pieces of tape, you will fold the fabric in a little bit less half, so that you leave a gap like this:
Then you tape up the sides,
cut a notch where it hits the top edge, and fold it in:
Finally, fold in half lengthwise and stick this baby under something heavy like a stack of books so that you have a nice crease in the middle. There you have it.
You can also dress it up with different colors and designs if you so wish. I've seen other ones where you put designs on the front, stenciled out with a knife. Just be creative! Have fun!