Instructables
Picture of Zombie Mouth FX Makeup
This was my first try at this, so I'm just going to briefly explain how I did it.

First I made a simple prosthetic by making a mold of my jaw with play dough, and covering that mold with liquid latex (I recommend you paint it on in thin layers with a q-tip. Don't want to ruin any brushes).

While it was drying, I covered my mouth with several thin layers of tissue and latex, then painted it black.

After it dried, I removed it from the mold and got all the excess play dough off. I also held it up to my face to take measurements, and to see if I needed to trim anything off.
But before I could apply the prosthetic to my face though, I had to make the teeth. So I grabbed my wax and molded small bits of it into tooth-like shapes (doesn't have to be perfect). The wax gets sticky though, so I put a very small amount of vasoline on my fingers to help moisturize it a bit.

After the teeth were made, I put them on by using small amounts of spirit gum and latex, and smoothing the wax into the prosthetic. Once all that adhesive dried and the teeth were decently attached, I painted them white with some face paint.

Then came the time to actually attach the prosthetic to my face. I held it against my face in the area where I wanted it, took some tissue and began to seal both corners to my skin with some latex. I then continued to layer tissue and latex until I was satisfied. After all that dried, I added paint, fake blood, and then I was done.

I have since figured out easier ways to do this, but it was just an experiment. I plan on uploading a step-by-step in the future, along with a full faced look using this same prosthetic technique.