Aluminum Sheeting (also called sheet metal) -- I use 26 gauge, which is about as thin as you want them to be. You could probably step up the thickness to a 24 or 25 and be fine.
Tin Snips -- these are the giant scissor looking things. They cut metal really well.
Some sort of adhesive -- I chose this household sealant because it was safe for food, and holds up well to water. You do NOT want to poison anyone and you do NOT want your cutter to fall apart when you wash it.
The tiniest little clamps you can find
Clip art or drawing the size and shape you want your cutter to be
** If you are making a trip to the hardware store and don't already have pliers...you should think about picking up some. But they are not necessary.**