Step 1: What you need
You will need the following things
a Polystyrene head (ebay)
10kg NewPlast modeling clay, this is a clay that is reusable like plasticine
20-25kg Crystacal R Plaster (this is a hard plaster, much harder than plaster of paris, do not use plaster of paris for this its too soft)
Jute Scrim (burlap)
Hot Glue Gun
Large Marbles or other round objects to make the key points.
1-2 litres of Liquid Latex
100ml Latex Thickener.
Talcum powder (Talc)
Plumbers Epoxy Putty (for mold repairs)
AirBrush Propellant or compressor.
Artists Paint Brushes.
Washing Up Liquid.
Tubs to mix paint in and store it for a short time.
Sheet Metal pieces
Depending on the mask your doing you may also need some foam, i found buying a cheap yoga mat to chop up works well, i used this to fill the dreadlocks.
Step 2: Sculpting the design
Take your polystyrene head and build up layers of Newplast so that it matches the dimensions of your own head or slightly larger as the latex will shrink slightly in the mold.
Now you can start sculpting your mask design, mine is the predator so i just added layers on and manipulated them to the shape i needed.
its important to note that you should try and keep things simple, if you have complex parts on the design you will need to cut these off and mold them on there own.