Hahaha I know, the teeth are horrific!!
I suppose I cheated in a way because I used an old mask as a base for my paper mache.
Here are some of the things I used including the old mask:
first I covered the mask in a layer or two of tin foil and then a layer of masking tape. Then I put plenty of layers of paper mache on using watered down glue and toilet paper. when I had enough layers on and it was bone dry I CAREFULLY cut down the center and took it off the old mask. Then seal it back up.
I used cotton wool to add the sores and the big lips. Styrofoam for the teeth and his rotten nose. I roughly drew out what I wanted before painting.
The hair was an old wig that I back combed and stuck on with hot glue. Painted with ordinary kids paint.