Prepare the silicone rubber by following the instructions on the container. Modeling clay can also be substituted for silicone. Make sure the consistency is thick enough to work with when you place it on your skin. Mix a little red and brown into the mixture. You want the silicone to match your skin tone.
Using the tongue depressor, spread the silicone across your skin. Spread as far as you would like the scar to reach. Use a small paint brush to feather the sides of the silicone so it will blend in with your skin. Then, take a toothpick and carve down the center of the silicone to form the opening of the wound. Feather the sides again.
Wait for the silicone or modeling clay to set on your skin. This typically takes 5 to 7 minutes, depending on the product.
Using theatrical make-up, color the wound. Make sure to keep the outside sections the same color as your skin color. Paint the center of the wound red to simulate blood. You can add more red paint to simulate a wound that's still bleeding.
Allow the paint and make-up to dry, and show off your new scar to your family and friends.