Mix 1 cup of corn syrup and a few drops of red food coloring and a drop of blue food coloring. Add chocolate syrup until you achieve the color and consistency of blood.
Paint liquid latex on a smooth object. Using your fingers, mold the latex into the shape of the scar you want. For a fresh scar, leave a hollowed out area in the center. Allow the liquid latex to dry, and use spirit gum to adhere the scar to your body.
Paint the scar with face paint. Use a dark red on the complete scar, and highlight the raised areas (where light would hit the scar) with a lighter shade of red. Dab light red around your scar as well, to add depth. For a healed scar, use a bit of pale maroon instead of red.
Add fake blood to the hollowed out area of your scar for a fresh, gory appearance.