And don't forget to vote for Horror Head in the DIY Halloween contest!
Step 1: Can You Draw Me?
In this case, rather than making a papier-mache shell over an inflated punch-ball balloon, I built the mask on a Diamond Comics corrugated cardboard shipping box - this takes far less time and works just as well, but it must be said that this method is slightly less cool.
Besides, even if I had taken detailed photographs, they can only help so much in terms of instruction. Essentially, this part of the process is sculpture, and you just have to make it up as you go along! I create shapes by wadding up newspapers and wrapping them with masking tape, then taping them to the box. It evolves organically over time, and I change it while I go, turning the head around to view it from various angles and just trying to make it look as cool as I can. The most important piece of advice, particularly if you're building on a box or balloon, is to do whatever you can to disguise the shape of your starting point. You don't want your mask to look like a box with a face, or a sphere with some pieces stuck on it... you want to incorporate that starting shape into the overall design and bury it beneath the face you're sculpting! On this mask, for example, I added the dome-shaped top and used another strip of cardboard to make a very angular, jutting chin, which helps to diminish the boxiness.
So, if you want to make an exact copy of my mask, then my first instruction is "MAKE A MASK OUT OF CARDBOARD, PAPER AND TAPE THAT LOOKS EXACTLY LIKE THIS."
Otherwise, start by experimenting with a way of sculpting the mask that is comfortable to you. It took me a while to find a method that worked for me, and everybody's different. Just bear in mind that the more materials you use, the heavier your final mask will be... and even though it's just flour and newspaper, the papier-mache will add quite a bit of weight.
You can see from the pictures here that I did not really start taking photographs until just after I started to coat the mask with papier-mache. The nails sticking out of his head posed certain problems, to ensure that they would stiffen and set at the correct angle. I tied the nail from his right eyelid to the nail protruding at the back of his head to keep it propped up while the flour-paste dried.