Soak a rock in bleach and rinse it, or wrap a clean rock in plastic wrap. This will prevent any bacteria on the rock from getting into your food.
Place your herbs, spices or grains in your bowl. If using a wooden bowl, double line it with plastic wrap before crushing strong-flavored herbs and spices. The goal is to prevent the bowl from absorbing these flavors.
Smash your food inside the bowl by bringing your rock down upon it. Continue until you have ground your food or have pulverized it to your liking.