Let’s start doing the ‘action line’ that includes the head and spine.
Here I divided the spine in three segments, for the neck, back and tail. The ‘tail’ is a bit more than the half of the total lenght of the spine, and the ‘back’ is a little longer than the ‘neck’.
The neck presents a soft ‘S’ shape, while the back is an arc. The tail wasn’t very flexible, but it continues the line of the back.
For the head, these simple shapes will be enough, but I always recommend to look carefully at a skull before drawing.
This is a very simplified diagram, the real spine is composed by several bones, the vertebrae, and it is divided in four segments: cervical (neck), dorsal (back), sacral (hip), caudal (tail).