The Torpedo Scull is a fairly fast scull. It's also used on the end of several figures like the jumpover or front walkout. To torpedo, start in your back layout just like you did for the Head First and Foot First sculls, but your arms should be above your head like this:
With your hands flexed so your palms face outward, make gentle figure 8s above your head. This should propel you feet first.
Remember, this scull is no different than any of the others in that you must keep your entire body extended. Think about stretching toward the opposite pool wall with your toes.
Keep your hips from sinking by squeezing your butt muscles up toward the sky (or ceiling if you're indoors).
Tilt your pelvis up toward your chest as if your belly button was hooked to the center flower on your bra.
If you still find that your legs have a tendency to sink, think about where your arms are. If you scull too wide, you lose a bit of the momentum and your lower body goes under. Correct this by keeping your elbows closer to each other.
It'll take some practice, but you'll soon get the hang of it and be sailing across the pool before you know it!