To make a rubberband gun, you will need 1X1 plywood. They sell it in 36 inch lengths at most hardware stores, but you only actually need about 26 inches.
You'll also need wood glue, a hammer, nails (we used the 1 1/2 inch ones with the very small head), 3 clothes pins, rubberbands, and (if you like) paint (any kind that will adhere to wood) and stickers.
You might want to get enough materials to make an extra, because the little ones sometimes have a hard time wrestling these guns away from the Daddies :-) (And in this case, Mama too!)
Enjoy!