Homemade flagstones are an inexpensive alternative to store-bought ones and can be customized to any size and shape that you want. Flagstone molds are often made of silicone and other expensive materials, but you can make them out of concrete so long as you take a few considerations into account. The form for your flagstones will be made of insulation foam board, allowing you to make many variations for more natural flagstones.
Cut the desired flagstone shape out of insulation foam board.
Bevel the edges with a sharp knife or hacksaw, so that the bottom of the flagstone shape is slightly wider than the top. This will help you remove the completed flagstones from the mold.
Use a wire brush to add texture to the top and sides of the flagstone form, if desired. Use light pressure to avoid tearing gouges in the foam.
Melt in surface cracks using a finishing nail heated with a lighter, if desired. These details should be shallow; they are only to give the appearance of cracks. Wear leather gloves when handling the hot nail.
Glue the flagstone form face-up into a plastic or cardboard box. The box must be at least one inch larger on all sides than the form.
Spray the form with three thin coats of acrylic enamel spray to seal the surface. Allow each coat to completely dry.
Brush the form and the interior of the box with a thin coat of petroleum jelly. It will act as a mold release.
Mix a small batch of ready-mix concrete with water according to the instructions on the package. The concrete should have as small aggregate as possible.
Pour the concrete into the box to the side of the form, allowing it to fill and cover the foam. Stop pouring when the concrete is an inch above the top of the form.
Allow the concrete to harden for several hours.
Remove the concrete mold from the box, and pull out the foam.