When making homemade soap, you need a soap mold. By pouring the hot soap solution into the mold, it solidifies in that shape. It's actually quite simple to make a soap mold, using a variety of commonplace items. Materials such as cardboard boxes, plywood, cookies cutters and metal cans can be used to make molds. While you can make a mold in any shape and size you wish, the easiest and most commonly used molds are circular, rectangular or square.