Back of head + neck

Once I was happy with the general shape of the face, I moved onto the rest of the head. First, I took a sphere and fit it to the size of the top of my head. I then rotated it to get a clear seam in the centre. I deleted the half that would eventually be created using the symmetry modifier, then used the paint selection tool to pick out polys to delete to make it match the face.

I matched up the vertices to the face and began extending the sphere down the back of the neck.

I continued extending to the top of the back and chest, and underneath the chin, meeting the symmetry modifier in the middle. I did encounter a few irregular polys during this stage, but I used the cut tool to fix them.

I then attached the back of the head to the face, and welded the vertices to complete the head, with the ears left to go.