The idea of this instructable is to explain the basics of how simple fake wounds are made. In the picture of the end result, the makeup and general colored area is focused in one spot, which makes it look a bit more fake than wanted.

This problem can be fixed in two ways:

One, create a more logical design for a cut. In this instructable the leg seems to look like it was scratched by a ferocious, infected, three-clawed beaver; smeared with mud from a lake, (which would actually make sense if there was a beaver around); and cleaned around every area except for the cut itself. Unless that's what you're aiming for, try to think of what actually gave you the injury before you make the design.
Two, adding more paint or makeup around the area makes the cut, bullet hole, or burn look dirtier. Even if your doing a small, simple cut, you have to keep in mind what the surrounding skin would look like if the cut was real. A deep cut on a finger might flood blood to one area while the rest of the finger turns pale. You don't want to be left with a single area untouched while the rest of your body is clean.

Supplies:
-Black, red, and brown acrylic paint. (For dirt and infection, it's nice to have a mixture of green and yellow on hand)
-A bottle of fake blood. (Found at nearly any costume shop, though it is a simple process to make your own)
-Scar wax (You an find this at practically every costume shop, too)
-Cutips
-Flour (For an alternative to texturing with an easily available material)
-A large needle
-Newspaper (Unless you enjoy mopping fake blood off the floor)
-Paper plate

Step 1: Creating the Base

-Open up the scar wax and scoop out a ball of the size in the picture.

-Smooth the clay to create a long, flat worm shape and lay it on your leg/arm, wherever you want the cut to appear.
(If you put clear nail polish under the area you put the wax on, it will be easier to wash off when you are done).

- Gently smear the edges on your skin. It's alright if the wax isn't smooth; you'll be roughing it up later.

Step 2: Making the Cut

-Using the dull side of the needle, draw a line through the middle of the wax.

-Pour a small bit of black, red, and brown paint on to the paper plate, and dip the end of one of the cutips into the black paint. Drag the tip of the cutip through the thick line. Do the same with the other two paint colors.

Step 3: Adding the Blood

-Put a small amount of flour on the paper plate and rub some in your fingers. Sprinkle the flour dust on the cut; this will help the blood and paint stick to the area.

-Pour a little bit of the fake blood on the paper plate. Mix it with tiny amount of black to make a darker, older looking blood. Dip the pointy edge of the needle in the mixture, and run it through the main cut. You can then fill the smaller cuts with blood, and feel free to add some fake blood without black paint in it to make it look a little more fresh.

Note- If you like how the cut looks by this step, theres no need to read the last two steps.

Step 4: Add Infection

-Add the green and yellow paint to the plate, and blend a small amount of the green into the yellow. Blend a small amount of the leftover red and brown paint in to the mixture.

- Dip a clean cutip into this paint mixture and dab it around the cut. Blend some of the blood on the wax into the new paint on the wax.