Set up a safe, efficient work area. Plaster production work is very messy, so cover the entire area with plastic tarps, including the floor. Lay out the pattern molds on the floor or a tabletop. Set up the work area so you have a source of room-temperature water and all tools at hand. Minimize walking because plaster dust "tracks" easily and is very difficult to clean off floors. Wear old clothes that can be simply disposed of. Wear rubber gloves; plaster is not caustic but it is difficult to remove from the hands, especially under the fingernails, when hardened.
Determine the ultimate use of the faux stones. Will they be affixed to a vertical set piece as a decoration, like the chimney of a fireplace? If so, remember to account for the weight of the faux stones in the design. Plaster of Paris can be heavy. Consider using Hydrocal, or dental plaster, to reduce weight, if necessary.
Create a selection of pattern molds. Use modeler's clay to sculpt several stone shapes and then create permanent "positive" molds from these clay shapes using plaster or Hydrocal. Another technique is to drape real stones with plaster-impregnated bandages like the type doctors use to form casts on broken limbs. When the "cast" dries you can pop the mold off the real rock. Closed-pore liquid latex rubber products can also be brushed onto real stones in many layers to form true rubber molds. However the molds are made, be sure to vary the sizes and shapes; seven to nine different shapes and sizes should be enough to mix and match and thus make them look more realistic. To save on production time, create as many copies of these pattern molds as possible. If you only have one copy of each mold, you will have to wait until each plaster casting is dry before you can reuse the mold.
Coat the inside surface of the molds with petroleum jelly or a commercial "mold release agent." Mix small quantities of plaster or Hydrocal using room-temperature water only. Avoid whipping the plaster while mixing to keep air bubbles to a minimum. Pour the mixed plaster into the mold, in batches, until the mold is filled. Imbed bits of chicken wire in the molds for extra strength. Let dry thoroughly. Plaster of Paris can take several hours to fully cure, or harden. Hydrocal will cure within an hour.
Carefully pop the mold off the plaster casting. Let it dry for another hour before painting.