Making your own stepping stone molds is a simple process: just build a wooden pour box, put your model stone in the bottom of it, and cover it with pourable mold-making compound, such as urethane. The question is -- what to use for a model? Here you are limited only by your creativity. Your model stepping stone can be almost anything you can fit in the box.
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The most traditional stepping stone look is that of flagstone, and they're the easiest mold to make. Just put a flagstone in your pour box and cover it with mold compound. If the bottom of your stone isn't completely flat, you'll have to set it in a bed of modeling clay so that the urethane can't run under the stone.
You can make stones that look like a cluster of bricks by arranging paving stones in the bottom of the pour box. If you want to simulate a grout line between them, you can mix up actual grout, lay it over the pavers and trowel the excess off. Let the grout harden before you pour the mold-making compound.
Place a 2- to 3-inch-thick slice of log in the pour box. It's a good idea to use a just-cut slice; this way the bark will still adhere tightly to the log. Seal the log with acrylic sealer and let it dry before pouring the mold-making compound.
For a whimsical touch you can make stepping stones that look like the prints of an oversized bare foot. Trace a large footprint on a 2-inch-thick piece of expanded polystyrene (e.g. Styrofoam) and cut it out with a hobby knife. Seal the foam footprint with clear acrylic sealer, then cover it with a layer of wax mold release. Don't skip this step -- unless it's properly sealed, the casting resin may melt the foam.