Make some foam cutting tools if you feel like it, a good sharp utility knife will do OK on this step. This link has a great variety of foam cutting ideas:http://wolfstone.halloweenhost.com/HalloweenTech/fotmak_MakingFoamTools.html
Polystyrene or styrofoam of some kind is carved into the shape you want to cast aluminum into. I used egg cartons, take-out containers, and dumpster scavenged foam. These foam pieces are exactly how the aluminum cast will turn out, so put some effort into this step.
First make two rectangular blocks out of the foam, about a half inch think and 8.25" by 3.75".
Print out a stencil or create your own for guiding and cutting a design on the 8.25" x 3.75" foam blocks. Here is this stencil link for use in Google Sketchup
. If you make your own design, you'll want ribs of some kind as part of it to strengthen the tile. The templates I made have two squares which are the ribs. Cut the features into the foam very carefully with a sharp utility knife or exact o knife. The depth of the imprint should be shallow, about 1/16". A polystyrene egg carton is about the right thickness, a meat tray is too thick.
One foam piece should have an negative impression in it and the other should have the positive impression using egg carton foam (see picture). The two pieces of foam should have some wiggle room when assembled to make room for the thinkness of the aluminum can that will be pressed when the die is finished. You'll still need to clean up the final cast with a dremel tool.
Now finally hot glue on a foam sprue on the center back of the block; this forms a channel for pouring the molten aluminum into the cast. I used a 3" long piece about .5" x .5". In the pictures you'll notice two sprues/vents but these do not work, use a single center sprue.