I wrote this article for The Makeup Artist Handbook by Gretchen Davis and Mindy Hall. Check it out! It's a great read!
http://www.amazon.com/gp/product/0240818946/

Making a silicone master mould:
To make my urethane moulds I start by making a normal life cast in plaster using alginate or silicone. We will make a master mould out of it.

Try to identify all possible undercuts that can be a problem in the mould making. Typical examples of these are shown in the picture.
There are several other undercuts that should not be removed. Like undercuts at the nostril or side of the nose. If these are removed your finished piece will not fit your actor as well as it should.

All the chosen undercuts are removed by filling them with plasteline clay. Try to mimic the original texture of the face on these parts.
All other defects on the plaster cast are also fixed up.
For this demo I used a plaster cast that I fixed up but quite often these days I never make a plaster cast at all. I instead fill the face cast with molten clay as you can see in my fake head tutorial. Check it out!

There are many ways to thicken your silicone depending on which brand you are using. I have found Poly fiber to be great as a filler.
Poly fiber blends into most silicones with ease and will not cause too many bubbles.
We will also use it later to thicken our urethane plastic.

On the finished corrected life cast we will create a master mould of the face. A negative mould of your actor means you can create many positive moulds to work on at the same time. You can make smaller moulds such as the nose or chin.
To make the master mould:
The first layer of silicone is very runny. Mix the silicone according to the info on the tin.
For this mould I used Platsil 10 (I tend to use it for everything) which has an easy 1 to 1 mix.
For the first layer I mixed 50 grams A with 50 grams of B.

As you can see I have drawn a red line on the board around the head. This serves as a guide on how far out my mold should extend.
Later we will be drilling holes through this part to keep the mould together. It is very important to get it wide enough for this.

Brush on a few layers of thickened silicone. I still mix 50/50 by weight but add about 10% Poly fiber to the mix to make it less runny.
You may use more or less any silicone brand for this.
To stay safe stick with the same sort of silicone that you will cast your final pieces in.
For this cast I used Platgel 10 from Polytek thickened with Poly Fiber II also from Polytek.

The finished silicone skin is about 1 cm thick and weighs about 300 grams.
The silicone should be fully cured before you go on to the next step.

Since the silicone is very soft and won’t hold its shape it needs to be supported on the outside with about three layers of plaster bandage.
Let the plaster set for at least 30 minutes before taking it apart.