This isn't too difficult. Here's a link that goes over the basics.
ALWAYS MEASURE YOUR AMOUNTS. DON'T WASTE!
Always use cold water when mixing plaster. First the water and then add plaster.
-Shouldn't take more than 1-3 minutes to slowly add the plaster.
-Let the plaster sit for a minute of two.
-Use your hand to squeeze out the lumps and gently mix for a minute or two
I learned the following formula that hobbyists use for making plaster molds.
1 qt. of water plus 2lb 14 oz. of plaster= 80 cubic inches of volume. It's a pretty forgiving formula and I've scooched a bit on both the plaster and water at times. Use a mold release on your mold (can be a very thorough spray if pam if necessary or a smear of Vaseline if you don't have mold release. Add the plaster to the water slowly as it helps avoid clumps. Mix as thoroughly as possible with your hand to break up clumps. It may appear too thin but it is fine. Then pour into your mold and wait until it sets up. As the plaster sets up a chemical reaction occurs making the plaster heat up. Only after it has thoroughly cooled remove the plaster from the mold.
Calculate volume by multiplying Width x length x height.
Below I'm also including a link to the formula the sculpture students use. This chart includes sample volumes with the corresponding plaster/water use.
Here is a link to a video going over the basics. Pretty useful.