Items you will need:
-Gelatin-skin (a great tutorial here --> http://www.zombiemaker.com/makeup/gelatine_skin_basics )
-Face and Nose Wax (or any sort of make-up wax will work)
-Toilet paper (2-ply)
-Eyeshadow in bruise colors (yellow, blue, green)
-Cream make-up (red, yellow, blue)
-Face powder (or Neutral Set)
-Liquid Latex (optional)
And some great music to listen to never hurts :3
Step 1: Getting Started
Step 2: Wax Time
If it starts to stick to your fingers, wipe them off on a dry paper towel. Getting your fingers wet while they're covered in wax will only end in tears.
Step 3: Toilet Paper Time
After you have as much as you want (seriously, go crazy), take the red cream make-up and a brush and start splotching the red onto the toilet paper.
Step 4: Still Coloring...
Then, mix together a bit of blue and yellow to make a nice green, and add that as well.
Step 5: Gelatin-skin Time!
Make a slit through the middle of the skin, first off, elsewise how will you see the tissue beneath? Don't make the slit once it's attached to your face. That's not a good idea, and you'll probably end up not needing that fake blood after all.
Using your Spirit Gum, brush it on all around where the gelatin-skin will be. Wait for it to be a bit tacky feeling before you slap the skin on.
Step 6: More Wax
Liquid Latex or the wax.
Now, I used both, and that ended a bit badly, since the wax peeled up the latex. I recommend just using the wax.
What for, you ask? Well, using the wax near the edges of the skin will help you blend it into your skin better, so it doesn't look like you have fake skin on at all!
Take the wax and pull it outward, away from the gelatin-skin and onto your face, so it starts to look a bit like this.
Step 7: Adding Color
Using that ghastly green color you made earlier, sponge it on with a make-up wedge. Be VERY careful around the edges where the wax is, because you might end up pressing the wax down against your skin, and you'll be able to see the edge of the gelatin-skin. We don't want that.
If it happens, however, put a bit of the green on your fingers and tap the wax to make it stand back up.
Experimentation is the best way to get the look you want.
Step 8: More Green
(If you're going for a less-long-since-dead and more recent zombie look, then use black, blue and yellow around the wound and a foundation of your skin tone to blend it.)
Step 9: Fake Blood
Get your fake blood (and make sure you're wearing clothes you can get dirty) and go wild.
I poured my blood from the cap of a water bottle straight into the wound, then used a brush for around the edges and such. You can do whatever you like!
Step 10: You're Done!
(Remember to make the rest of your face more dead-looking, unlike me!)