The first step is to design and create a mold. Then you can make as many matching stepping-stones as you like.
1. Roll oil-based 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 12 x 12 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. 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 to 25 coats for reusable molds). When thoroughly dry, remove mold from the clay model and boil for 10 minutes to strengthen the latex.
3. Make a shallow square box to hold the rubber mold. For the box frame, use 1 x 3-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. Prepare ready-mix concrete following package directions. A 60-pound bag makes four 14-inch-square stepping-stones.
5. 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. 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. 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. Use a brick to smooth rough edges and to remove imperfections in the stone. Allow the stone to cure for one week.
9. To color a stone, dip it in a bucket filled with a mixture of equal parts of 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.