I was taught this several years ago by Bernard the Cunning Artificer.
I do not class myself as an artist by any stretch of the imagination, so if I can do this, so can you.
(This head is not much over an inch high - if you make it larger, it will be easier to sculpt.)
Step 1: Materials and tools
Something to model with.
When Bernard taught me this, we used a special modelling wax, but it will work with most clay-like materials, especially if they can be smoothed off. I used Sugru* for this demonstration, partly to see if I could, but mainly because I came across a pack that was almost expired, and, er, needed to be used.
Something to mould the material.
You will mostly be using your fingers, but you can also use other tools for the finer points. Fine screwdrivers, cocktail sticks, bamboo skewers and penknives are all useful. For the Sugru, a damp knife-blade proved to be a very effective tool.
Some modelling materials can make a mess - keep something to hand to clean tools and work surfaces, just in case. If you are using clay, a damp cloth is ideal.
Sugru is especially unkind to unprotected surfaces. I used it on a sheet of sacrificial waxed paper, over a cutting board.
*Sugru does not yet come in realistic flesh colours. I made the brown here by mixing together two sachets of red, two of yellow, half a black and half a white. I might have used more white, except I wanted to keep some for the eyeballs. If you want to play with mixing Sugru colours, see the official site, or read the handy little pamphlet that comes in the pack.
Step 2: Skull.
Roll half your material into a roughly egg-shaped, or slightly oval shape. This will be the main part of the head.