Liquid latex is a rubber compound made of rubber, ammonia and water. When the water and ammonia evaporate, the remaining rubber dries into a solid form. Liquid latex has been used to create wounds for decades, and is available at any local costume shop. By using the latex as a base and building the wound out with cotton and tissue, you can create some horrifying results.
Research wounds to decide what shape and size you want to create. Some basic wounds are lacerations (jagged, torn cuts in the skin), punctures (bullet holes, stab wounds) and abrasions (wounds where sections of skin are torn away).
Paint some liquid latex onto your skin with a cotton bud.
Pull large chunks of cotton from the cotton balls and use it to build up the sides of the wound, pressing the cotton into the wet latex. The outer edge should blend down into the skin, while the inner edge should be raised to help make the wound look deeper. Don't worry if the cotton is a bit lumpy, it will be covered up.
Paint another layer of latex onto the cotton. If needed, add additional layers of cotton until the wound is as large as you want. When the wound is built up, add a top layer of latex.
Tear the sides off of one or more tissues to create jagged edges, which are easier to blend into the skin than straight edges. The tissue will form a smooth skin to go over the cotton.
Lay the tissue onto the latex-covered wound. Bunch it up a little inside of the wound for texture. Smooth the edges into the skin and tear away any excess.
Paint a final layer of latex over the tissue, extending it slightly onto the skin in order to ensure that everything is secured in place.
Paint the wound with cream make-up. The colours will be based on your research, but you definitely want a skin colour that matches yours, a dark red for the centre to make the wound appear deeper than it is, and a lighter blood-red colour for the meaty areas of the wound.
Add stage blood to the wound to finish the effect.
If you have a choice of latex brands, it's a good idea to get whichever is thicker. Thicker latex stays put better than thin varieties.
Do not get liquid latex into your hair, brushes or clothing. It will never come out again. Many types of red makeup and stage blood stain. Be careful around your clothes.