After cutting out the skull patterns in 5-6 oz leather, I handstitched the pieces together using waxed thread. After wetting it I pulled the skull over the head form to let it dry in the right shape.

Here is a rear view of the skull with the stitching almost complete.

After drafting the pattern for the brim, I cut it out and stitched it onto the skull. Then to make it a bit stiffer (
and to give it a finished look) I sewed on leather piping (
shown here partially done).