This tutorial shows you how to get a good zombie look without using anything you don't already have!
For this makeup you'll need:
-an eyeshadow pallet
-red and green food coloring
Step 1: The Base
Squirt some craft glue onto a piece of paper, and use a q-tip to apply onto areas of the face. These will become the smaller rot/wounds.
Peel apart 1 ply from a tissue, and rip it into a piece the size of the palm of your hand. This will be a large rot/wound area. Apply a layer of glue to where you're going to apply it. When it's on your face, bunch it to look like peely, rotten flesh. Go over the edges of it with the gluey q-tip to make sure the edges stick to the face.
Once all that is dry-ish, go over your whole face with foundation (powder will probably work better). Don't forget to pale your lips!
For the smaller rot/wounds without tissue, use tweezers to peel a hole or two in the middle of the glue, to give the appearance of a hole in the skin.
*N.B.- Really make sue you stick the edges to your skin. Other wise it won't blend in.