First you need a pattern of what you want to cast. You will need two hearts. Mine are about 5" tall and 3/4" thick. Start with a piece of foam insulation. You want the pink or blue stuff that they sell at the hardware store. It's possible to use white beadboard but it's crumbly and won't give as good of a resolution. I cut out my heart patterns on a cnc router (I got the cnc bug from this excellent instructable: www.instructables.com/id/How-to-Make-a-Three-Axis-CNC-Machine-Cheaply-and-/
but ended up using plans from here:www.solsylva.com/
which ended up being a two year diversion) You certainly don't need to use a cnc router to make your pattern. A saw, knife, file etc. will do just fine. I cut out the heart in two pieces and glued them together. Make sure that you add two sprues off the top of the heart. This is where the aluminum will enter and exit the mold. It took about 20 minutes to cut each piece. The ridges in the pattern are because I cut it out at a low resolution. I was too impatient to wait longer for a smoother cut and I thought the they looked interesting. I uploaded the G-code if you want to use my pattern.