Place your molds on top of your mat to determine how to cut out your stamps. Trace the shape of each mold. Carefully cut out each shape using a utility knife with a sharp blade. Trim as necessary to fit within the mold.
Lightly spray rubber stamp with cooking spray and set aside.
Mix sand/topping mix and water until you achieve a thick but pour-able milkshake-like consistency. Transfer mix to plastic mold.
Gently push the stamp into the cement. Set someplace warm to cure for 8-12 hours.
Gently pull the stamp upward. If it removes cleanly continue to remove. If the cement is still too wet/loose, push the stamp back into the mix and allow a few more hours to cure. Once the stamp is removed clean up any rough edges using rough sandpaper or a chunk of hardened concrete.
Remove stone from the mold approximately 24 hours after pouring. Set in a warm place for several days to fully cure and harden.