Create your sculpture out of modeling clay, unless you plan to cast a found object. Relief sculptures, which are flat on one side, are much easier to cast than full-dimensional sculptures. If you choose a full-dimensional sculpture, use a section of thin wire tied to two dowel rods to cut it in half lengthwise. You will create two molds, one for each half.
Coat the finished sculpture or found object with petroleum jelly. This will keep the object from sticking to the mold. Spread the petroleum jelly as thinly as possible so it does not alter the texture of your mold.
Fill a leak-proof container with about 1 inch of pourable mold rubber. Place the sculpture in the container. Fill it with pourable mold rubber until it reaches the flat back of the sculpture or the center cut, if you are casting a full dimensional sculpture. Let it sit overnight to allow the rubber to cure.
Pull the mold off of the clay sculpture or found object. Begin at one corner, gently bending the mold away from the cast object. Continue this process with the other corners of the mold, alternating corners until you have freed the mold.
Coat the mold with two thin layers of liquid latex before pouring concrete to cast your decorations. This will keep the concrete from adhering to the mold.