Use two parts plaster powder to one part water. Most plaster packaging instructions say to mix water into plaster. On the contrary introducing plaster powder to water will minimize the amount of bubbles. So start with water and gradually add plaster. Note- plaster powder shrinks as it soaks, you'll use a lot more then you think so be supplied with a big bag before you start.
Step 2: Stir
You can use a stir stick of some kind but I use my hands to be able to brake up all the lumps fast before the mixture starts to set. Be thorough and add a little more water or plaster until it's smooth and the consistency of pancake batter. If there are bubbles you can tap the bucket lightly on the work surface and watch them raise to the surface and pop.
Step 3: Imbed wax sculpture
Gently and slowly insert your wax piece into the plaster mixture. Sculpture down, sprues up. Sprues should remain visible on the top surface. Your piece should not reach the bottom or any walls of the container. Hold your piece hovering in position until you feel it's being supported by the surrounding plaster.
Step 4: Allow time to set
Allow the plaster to set completely, the longer the better ( I left it overnight).
Step 5: Pop the mold out of the container
For one piece I used a plastic container. For the other which didn't fit into a plastic container I improvised with aluminum foil. Remove whatever you used as a holding vessel and get to the bare plaster mold.
Step 6: Bake the mold
Even though the plaster mold appears to be dry, there is moisture trapped in it. In order to dry it out and pre-heat it before melting the wax out use a conventional oven and very slowly increase the temperature. Heat shocking it will cause the mold to crack so be patient. Follow my next instructable on burning the wax out of the plaster mold for lost wax casting for further instruction. Have Fun!