Mix a black flour paste. Place a couple tablespoons of flour into a custard cup and add water a trickle at a time, mixing with a fork until you have a paste about the consistency of toothpaste. Add one drop of black food coloring at a time, still mixing, until the mixture is deep black.
Prepare the area of skin where you want the scar. Shave or wax any hair and apply a coat of petroleum jelly or cold cream; this will keep the latex from sticking painfully and becoming difficult to remove, later.
Apply a layer of liquid latex to the skin. Use your finger or a disposable craft brush to apply a thin layer of latex over the “burn” area. Let dry (this should take roughly 10 to 20 minutes, but you can speed it up with a hair dryer).
Add texture to the latex. Tear some small strips and scraps of toilet paper and soak them in liquid latex. Apply these over the wound area, scattering them across the surface (they'll stick as they dry and set up). Don't worry too much about where you put them or how you lay them out, just spread them around. Let dry.
Apply fake blood to the wound. Smear thick blood over the entire surface as thickly as you can without having it drip or fall off.
Blot black flour paste into the wound. Apply by dipping your finger into the black paste, then dabbing into the red blood in the wound. Scatter blotches of black throughout; the more severe you want the burn to be, the more black you should use. Let dry (drying times will vary according to the brand formula; again, use a hair dryer if you want to speed things along).
Spray the fake wound with hair spray. This will help keep some of the flour paste and blood in place.