Place your finished mermaid tail sculpture in a protected work area. Make sure that all of the finishing details are on the sculpture, such as scale work and other sculpted designs. Spray the sculpture with Krylon Crystal Clear or another clear sealing spray so that it holds the details better. If you are working with solid, dried clay and not soft plasticine, coat your sculpture with Vaseline. Let the coating dry completely.
Determine the center line of the piece, going from the tail top, around the fin, to the other side of the tail top. Build up a thin wall of modeling clay along this line to prevent your plaster from spilling over onto the opposite side. Make sure that the wall is at least as tall as you want your plaster mold to be. If you do not want to make a two-part mold and will be casting in a soft material like latex that can be pulled away from the sides of the mold, skip this step and make a one-part mold.
Mix up a small batch of molding plaster in your bucket by adding gypsum powder to water until it has a thick, creamy texture. Paint a layer of plaster onto one side of your mermaid tail sculpture. Start with the side that is facing up, and paint all the way to the modeling clay. Try to have as few bubbles as possible, as they will compromise the strength of the mold. Let this base coat dry completely.
Mix up a larger batch of plaster and apply a second coat to the mermaid tail. Over this wet coat, lay strips of burlap and let them soak into the mixture to strengthen the mold. Paint a third coat over the burlap and let it dry. Continue on in this way, adding layers of plaster and burlap until the mold is as thick as you want it to be. Remember that the thicker the mold, the stronger it is, and that large objects will need stronger molds. Repeat this whole process for the other side of the sculpture when the plaster is dry. When finished, let the mold cure for six to eight hours.
Remove the modeling clay and pull the two halves of the mold off of your mermaid tail sculpture. If you have made a one-part mold, you will need to pull the clay out in pieces. For the two-part mold, clean the inside free of clay and strap the two halves together with a piece of rope or rubber bands, then stand the entire mold upright with the open end at the top. You can stand the mold upright by leaning it against another object, but you may want to build a wooden frame around the mold or place it inside of a box so that it does not tip.