I fell in love with Silurians the moment I saw them. When The Doctor peels off Alaya's mask in "The Hungry Earth", I had the same reaction he did. "You are beautiful!"
I resolved then and there to recreate this makeup. But, it's not exactly a simple design. There is a lot going on, and, to make things more difficult, there are notable differences between the designs for males and females, and most of the really good photos of the makeup process were, of course, on males.
Fear not, my fellow Silurian costumers, I'm here to help. First, let's take a look at the basic design.
The Silurian makeup comes in three pieces. The first piece applied is a thin layer of scales that is applied across the back of the neck, which you can see in the second picture above. There doesn't seem to be much detail in this piece, just your basic scale pattern. It stretches around the neck toward the front, to about the line of the cheek.
The second piece is the cowl. This is the main part of the makeup, and is made of a thicker, rigid rubber, and is almost like a helmet. These pieces simply slipped onto the head and had a bit of adhesive applied to the front of the piece where it connects to the face.
The last piece is a silicone face appliance. This piece was made by having the actor wear the cowl while having their face cast, so the makeup crew could then sculpt the facial appliance to exactly match up to the seams and details of the cowl. You can see the process in this article by The Sun.
These three pieces were the same for both male and female Silurians. The cowls seem to be a bit of "one size fits all", since during the Doctor Who Experience makeup demonstration, it was said that they "found a cowl that fit [the actor] pretty well", which says to me that they were not custom fit to the wearers.
There are three main crests on the head, one down the centre top of the head, and then one to either side. There crests have scales running up the length of them, with small scales outlining each fin. Wider scales fill the space between each crest.
There is a triangular area of wide scales on the forehead which taper down toward the nose.
The scales along the bridge of the nose are wide and flat, with two rows of small scales between the forehead triangle and the nose bridge.
There are large, round scales that move along the cheek, starting with small scales at the edge of the mouth, and gradually becoming larger and more pronounced as they move toward the back of the head. These scales stand out slightly from the rest of the face, creating a raised ridge along the cheekbone. There is also a large, round, flat patch along the jawline on either side, which is slightly larger on the female.
There are four ridges in the back, the top two having a bit of an upward dip in the centre, and the lower two being straight across the bottom. The scale pattern here is diagonal, with very little variance in size.
Colouring varies from character to character. Restac has a lot of red/brown colouring along her major ridges - the top three crests, the cheek ridges, the forehead triangle, and the jaw patches. In contrast, her sister, Alaya, has almost no variance in colouring. Her highlights are more a paler shade of green, with a very light hint of yellow here and there, and a slight touch of red on her forehead and fin. Malohkeh has very similar colouring to Alaya, though with more pronounced yellow tones.
Now that we've gone over the basic pieces, let's look at the differences between boys and girls.
The first thing that you notice is that the crests along the side of the head are longer in the female. They extend farther back and slightly inward. The crest on the top of the female head is also longer and more flared, with a bit of a fin in the back third of the crest. The male's top crest, on the other hand, looks to be entirely made up of scales, without the fin detailing on the back. The side differences can really be seen well during the application process.
The back of the head is also a bit different.
The female ridges extend out farther from the head than the male ridges do, and are more flared. You can also see how much farther the crests on the side of the head extend back on the female, sort of wrapping around the head to protect the back ridges. The males crests, on the other hand, are comparatively short, and extend straight back with very little of an inward curvature.
So, there you are! A very basic breakdown of the Silurian makeup. There are, of course, a million details to these appliances, and I've only gone over some of the very main features of the look here, but it's enough to get you started so you can start your own sculpt. Here are a couple of other resources which will help you with your own Silurian look.