Here are some images with short descriptions on the sandcasting process. See the Sandcasting Materials Page for where to get all that you see here!
Step one: load one side of the flask with sand! I use Petrobond; the stuff is basically fool-proof and reusable. LOVE IT!
Step two: flatten the sand out (pound it flat using a board), then hit the facing of the sand with a release agent: talc or graphite. The goal is to keep anything from sticking to it! Then, press your object in; carefully and evenly!
Step Three: hit everything with graphite or talc again and put the second flask on.
Step Four: With the sand in the top flask, use a rubber mallet and a board (or whatever serves) to pound the crap out of it and really tighten the sand into the object and the flask.
Step Five: Ready to remove the main tool (or piece being copied).
Step Six: Prep the mould and pour the metal in (this is a whole other process that requires its own training and experience).
A day’s casting with a few different alloys!
Theo's Metalworking Page 