Clean the area of the skin where the scar will be made with alcohol. A cleaner area allows the fake burn to adhere to the skin better.
Tear strands of cotton from the cotton balls and firmly adhere it to the skin in patches with liquid latex. Randomly stick the strands of cotton all around the area of the skin where the intended burn is to be located. Allow scales or flaps of cotton to form randomly all over the burn area. Think of a slice of pizza with the cheese peeled off. Allow the latex to fully dry.
Pour fake blood all over the cotton and between the empty areas. Apply some black paint or make-up in some spots of the cotton to create the appearance of burned and charred flesh, like Freddy Krueger from "A Nightmare on Elm Street" fame.