Don a pair of long pants and closed-toed shoes to prevent injury to your legs and feet. Sharp pieces of stone can tear your skin to shreds if you are not properly protected. Wear a pair of safety goggles to prevent stone chips from entering your eyes, as well as a good pair of work gloves to protect your hands from hammering friction that can cause blisters and cuts.
Place your larger stone pieces on a hard, flat surface. Do not place them on grassy sod because the force of your hammer will cause the stones to bury into the ground instead of break.
Hold your sledgehammer with both hands. Place your dominant hand in front of your less dominant hand with at least 6 to 12 inches between the two. Lift the sledgehammer over the shoulder corresponding with your dominant hand. Your dominant side is strongest.
Swing the sledgehammer down onto the stone, using your dominant hand's strength to increase the force of the blow. Repeat until your stone is broken up into small pieces and you can fit the stones into a small bowl.
Place one to two stone pieces in your mortar bowl. Apply pressure to your pestle as you place it on top of each stone, gently turning to reduce the stone to fine powder. Do this for all your stone pieces. Larger projects may require the use of industrial machinery for crushing. You can rent these machines through your local home improvement or gardening store.