For the curved parts of each vertebra I firstly marked sections at roughly 1.25" increments. Where I had made these marks I cut a "V" shaped groove around to begin the shape of the vertebra. To make the curves between these I simply used a gouge to cur around the stick. The gouge I used is around #5 sweep, I don't know the exact size because it's a home made gouge. These curves should be around 2/3'rds of the way around the stick, leaving space for the bones that stick out of the spine.
It'll probably take a few brews before you carve them all!