1. Create your design. Roll oil-base modeling clay to 1/4-inch thickness on a work surface. Draw desired patterns on paper and place them on the clay. Cut around each pattern with a knife. Place clay pieces on an uncut copy of a stepping-stone size paper pattern (about 12x12 inches), which has been glued to a plywood work board. Sculpt a design model by shaping and joining clay pieces. Spritz with nonstick cooking spray.
2. Prepare the mold. Brush a thin layer of mold latex over the surface of the clay model. Apply slowly to prevent air bubbles. Dry overnight and repeat the process until the mold reaches 1/4-inch thickness (20-25 coats for reusable molds). When thoroughly dry, remove mold from the clay model and boil for 10 minutes to strengthen the latex.
3. Build a frame. Make a shallow square box to hold the rubber mold. For the box frame, use 1x3-inch pine furring strips cut to the size of the mold, and secure strips with screws. Use a square of Masonite for the bottom of the box. Insert rubber mold, design side up, in box.
4. Mix the concrete. Prepare ready-mix concrete following package directions. A 60-pound bag makes four 14-inch-square stepping-stones.
5. Cast the mold. Scoop just enough concrete into the wooden box to cover the design (about 1 inch thick). Place a piece of chicken wire, cut smaller than the mold, over the concrete for reinforcement. Continue pouring concrete until the box is filled.
6. Allow to cure. Smooth the top with a trowel and tap the box on the work surface to remove air bubbles. Allow the concrete to set for 24 hours out of direct sunlight.
7. Unmold. Loosen screws on the box, invert it, and gently tap it to release the stone onto a cushioned surface. Gently remove rubber mold from the stone. (If the stone is left in the mold longer than 24 hours, it will be more difficult to remove and may tear the mold.)
8. Smooth the edges. Use a brick to smooth rough edges and to remove imperfections in the stone. Allow the stone to cure for one week.
9. Add color. If you'd like a colored stone, dip it in a bucket filled with a mixture of equal parts exterior latex paint and water. Or, use a large paintbrush to apply the diluted paint onto all sides of the stone. Allow it to dry for about one hour.