For the tail vertebrae, arrange each of the eight pieces from smallest to largest. The smallest piece goes into the slot at the far end of the tail, while the largest piece goes in the slot closest to the legs.
When you have the tail finished, you can add the back vertebrae. There are six pieces that we will arrange from smallest to largest with the largest piece inserted in the slot closest to the legs and the smallest nearest the head.
The third vertebrae from the head should be the piece pictured in the blue box to the right. You'll notice that this piece has two additional slots to place the arms.