To make a fake stab wound, latex "skin" has to be built up over your own skin, gashed and finished with fake blood applied to give it a realistic look. Fake wounds can be applied anywhere on the face or body, but you may want to practice on an area such as your leg so that you will have both hands free to work and won't need a mirror.
Apply liquid latex about 3" long with a cotton swab to clean, dry skin over the area where you want the wound. Allow it to dry for about 5 minutes. Apply a second layer, making it just smaller than the first. The more layers you add, the deeper the wound will be.
While the top layer is still a bit wet, use the plastic knife to make a gash in the latex. Allow it to dry for another 5 minutes.
Gently use the plastic knife to pull the gash open.
If using make-up, apply it with a sponge to blend the latex with the skin. Some liquid latex products match some skin tones, so this may not be necessary. Blend in some purple, red or black make-up to give the wound a discoloured, bruised look if desired.
Apply fake blood to the gash with a toothpick. Add enough so that it starts to drip, but don't overdo it.
Liquid latex can be found in most costume, Halloween and theatrical supply shops.
Do a patch test on a small area of skin to make sure you don't have an allergic reaction to latex before you start.