Draw multiple sketches of your three-dimensional mold from many different angles. Draw the sketches full scale if possible. Get a read on the size of your mold.
Purchase an appropriately sized high density block of styrofoam from an art supply store. For large scale projects, you may want to purchase a hot-wire cutter from a hardware store. The wire heats up and cuts through the styrofoam easily.
Take a pencil and lightly sketch the outline of your mold on all sides of the block. Shade different areas of the sketch as a topographical guide.
Use a sharp knife, hacksaw, or hot-wire cutter to cut into the styrofoam. You want the blade to be as sharp as possible so change the blades every eight cuts or so. It is a good idea to wax the edges of your knife before every cut.
Cut off small chunks at a time. Leave about 5/8-inch of styrofoam off of your mold so you can sand in the details.
Use coarse sandpaper to get the final shape of your mold, then use fine grain sandpaper for a polished finish. You can paint onto your styrofoam mold with acrylic and glue styrofoam pieces together with a low temperature glue gun.