The idea is to fill (and pack) the bottom one with "green sand" (a special casting sand which sticks together well) around the mold of whatever it is you are going to cast. Level it off flat. Then dust it with talcum powder to allow the two pieces to separate easily Then put the cope on top and add sand from the top to cover the top half of the mold. You add holes and channels to allow the pour to enter and exit, and then you are ready.