Steps for Mixing plaster
Plaster hardens by a chemical reaction with water. (Hardened plaster cannot be dissolved or softened by soaking it in water.) Once you start the chemical reaction by mixing you cannot add more plaster to the mix. It is important to get the right ratio of water to plaster. This is a weight ratio, but artists have come up with The Island/Continent Method which might be a helpful way of approaching it for you:
1) Estimate how much plaster you will need and put that much water in the bucket. Remember: Never fill the bucket more than 1/2 full of water otherwise it will overflow when you put in the plaster. Get a bigger bucket if 1/2 is not enough or mix two batches. Plaster is relatively inexpensive and the problems caused by running short are not worth the few cents saved.
2) Put the amount of cold water that you have so diligently estimated you need into your mixing bucket. Never put the plaster into the bucket first; always put the water in the bucket first, and then add plaster to the water.
3) Scoop up plaster with your hand. Wiggle your fingers and shake your hand to make the plaster fall through your fingers into the water in the bucket below. This process prevents large clumps of plaster from falling into the water. Keep sifting the dry plaster into the water with your hand. Work quickly and spread the plaster all around the surface, evenly. Plaster should fall like gentle rain on the water surface, and then sink out of sight. You don’t want a lumpy plaster mix. Do not mix the wet plaster or disturb it in any way while sifting it in or this system of measurement will not work. Keep sprinkling plaster into the undisturbed water. Eventually you will notice little islands of plaster forming above the water surface.
4) Keep sifting the plaster in until you end up with one large continent (with no lakes or volcanoes) that does not sink. You now have the proper amount of plaster in the water and are ready to start mixing.
5) Wash the plaster off your hands by dipping them first in the blue buckets filled with water near the sink and then use the sink. If you use the sink first the plaster will harden in the sink!