Clean the skin where you want to apply the cut. If the area is hairy, shave first.
Apply the spirit gum to the skin. Spirit gum is a makeup adhesive available at any costume or makeup supply store.
Pull apart some cotton balls and start placing them on the spirit gum. Create a thick layer of cotton on the spirit gum until you can't feel the adhesive underneath.
Color the cotton with fake blood. This will later have the appearance of the flesh underneath the cut.
Apply castor oil to the red-colored cotton balls.
Rip a small sheet of toilet tissue that will completely cover the red cotton balls.
Place the toilet paper on top of the red cotton balls.
Apply liquid latex to the toilet paper with a makeup brush. Cover the entire area and apply latex around the edges to make sure it stays attached.
Apply more paper and latex until you have completely covered the cotton balls with a thick coating of latex.
Use a hair dryer to dry the latex. This may take several minutes.
Apply powder to the latex appliance.
Take a makeup sponge and start gently applying flesh-colored makeup to the latex appliance.
Slice open the latex lining in the shape of the cut you wish to create. Cut only deep enough to expose the cotton lining underneath. Be very careful not to cut too deep, as you do not want to injure your actor.
Apply fake blood to the cotton balls and the outside of the cut. You can paint some blue areas around the cut to simulate bruising before adding the blood.
Reattach any seams by applying spirit gum.