As I recall, the Corn Snake Manual's examples of tails were from the underside. I've always had better luck sexing from the side. YMMV.
I find the vent and then gently straighten the tail. Locate the halfway point between vent and tail tip. Run the eye from vent to halfway point. The females' tails have a fast taper and are less than half as thick at the halfway point as at the base of the tail. The males' tails have a slow taper and are half or more than half as thick at the halfway point as at the tail base. Remember, this is from the side, not the venter.
Practicing on snakes of known sex improves accuracy. Accuracy is greatest with adult snakes and poorest with neonates. Good luck.