Mix your lime water. Fill a bucket with water and slowly add the appropriate amount of lime powder, per the instructions on the label. Use a spoon or long stick to stir the mixture (since the lime and plaster of Paris will likely ruin anything you use to stir it, use something disposable). The lime will partially dissolve in the water, leaving you with a white, opaque "lime milk." Wear protective goggles and gloves whenever you're handling the mixture, to protect your skin and eyes.
Add plaster of Paris powder to the lime milk. Again, follow manufacturer directions in regard to safety and measurements. Do not dump the plaster of Paris into the bucket. Instead, sprinkle it in as you stir the lime milk with your stick or spoon, to avoid lumps of partially mixed plaster in the mixture.
Stir the mixture until you have a smooth, lump-free consistency. Use your plaster of Paris as soon as it's adequately mixed. Although the plaster and lime mixture will not harden immediately, it will begin to set as soon as motion stops. As soon as the mixture begins to harden, it will be unusable. Use it immediately to ensure that you have a fully transferable medium for pouring or spreading, which will give you best results.