I found a generic head model off the internet and added it to image planes for reference.
Using the create polygon tool and the front and side images, I began to sculpt an eye socket and the basic shape of a mouth.
I extruded the edges around the nose and began to sculpt the forehead.
Again following the geometry from the image planes made the chin.
I then combined the two pieces of mouth geometry and the rest of the face together and began on the nose.
Basic head shape ready for more detail, inserting edge loops etc.
I mirrored a proxy mesh so I could see the updates in real time when adding definition to the eye socket, cheek bones and mouth.
I duplicated the face and used the soft modification tool to warp the extra meshes into sad and happy expressions. Next up, texturing.