First, you must find a subject for your wall plaque reproduction. As a public service, many educational institutions present photographs of reliefs from the ancient Greeks and Romans. Since the two cultures were intermixed, many Greek gods were also Roman gods. After you find a picture, print it out.
Cover a thin piece of plywood, cut either in a square or rectangle, with a two inch layer of modeling clay. The wood and clay should be sufficiently large enough so your picture fits in the clay area. Lay your picture on the clay, and using a pencil, press lightly on the outlines. Remove the paper. Your clay should have the thin outlines of the figure. If you goof up, don't worry. you can resurface the clay with a putty knife, and start over.
Carving the clay is the hard part, since the skill as a sculptor has to come from within. Obtain a set of clay sculpting tools. These are relatively inexpensive, and available at any artist supply house. Deepen the outlines, until the relief carving is made. Frame the carving with cardboard or wooden sides, about 4 inches tall. After carving, clay sculptors at Laguna Clay, citing renowned sculptor Anthony D. Bulone, recommends spraying the clay down with three coats of orange shellac. Pour plaster onto the clay. Shake the mold gently to seep the plaster deep into all the crevices. Allow the plaster to cure.
After the plaster has cured, gently remove the clay. Be very careful, since the plaster is soft. Use wooden craft sticks or soft brushes only. After all the clay is removed, spray the molded side of the plaster with polyurethane. Apply three or four coats, in very thin layers. Allow the polyurethane to dry.
The relief sculptures themselves are made of plaster. Apply mold release wax to the molded side of the plaster. Laguna Clay recommends dusting the plaster mold with talcum powder first. Allow the wax to cure, following manufacturer's directions. Pour wet plaster into the mold, and allow to cure. After curing, gently remove the sculpture. Blow hot air in between the relief and the mold with a hair dryer, to soften the wax. The mold can be used many times over, so you can mass produce wall relief sculptures.