Mix the Vaseline and flour together until it blends well. You should have a clay-like substance when it is thoroughly mixed. Stirring can be done with the hands or with an instrument, such as a popsicle stick.
Add cocoa powder slowly to the Vaseline/flour mixture. This will create the proper skin color.
Apply the mixture to the area where you want the wound to appear. Smooth out the edges, but leave the center rough and open so the skin appears cut.
Use fake blood inside the open wound area and spatter or trickle some around it, depending on the type of wound you are trying to portray.
Decide where the fake wound will be and apply liquid latex on the skin as foundation to build the wound.
Mix oatmeal and latex in a cup. Apply this on top of the foundation after it is dry. The oatmeal will create a ragged look that mimics an infected wound. Smooth edges to suit the look you want and dry with a hairdryer.
Apply a combination of corn syrup and food coloring to create fake blood This mixture adheres to the oatmeal and latex combination better than store-bought fake blood.
Melt gelatin in the microwave slowly at 10-second intervals. Let it cool. Apply to the area where the wound will be with a popsicle stick. The gelatin is useful in making a wound structure, but can also represent bones as well.
Work the gelatin to smooth it out around the edges. Repeatedly go over areas that should be rough with the popsicle stick to create texture.
Apply fake blood in the open areas. Spatter or trickle as needed.