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.
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.