This image I very crudely drew in 5 minutes may help you understand (or perhaps confuse you more). Here's the explanation of how
I do it. I may be doing it wrong, but it works alright. Anyway....
This is assuming you're right handed. Look over your left shoulder. That's where you're aiming. Turn your left foot to point in that direction. At this point you should have the sling in your right hand, loaded and ready to fire. Now quickly (speed is essential to this method) move your arm in a complete arc from behind (assuming the way you're facing is forward) your right leg, all the way over your head to the left of your left leg (do this motion a few times without the sling to get a feel for it). You'll probably want to step forward with your right leg as you sling (it may make the motion easier). Be sure you use a sling short enough that when you hold it by your side it doesn't touch the ground. Don't forget to release when the rock it at the top of the arc! Also, don't expect the rock to travel far at all. It only goes about 20 feet maximum when I do it using tennis balls, but then again, I'm rather weak. Take a look at the pictures below to help understand this text.
Oh yeah, I forgot to mention: your shoulders should twist from facing right to facing forward as you sling, and your left arm will probably move behind your back. It may also help to point at what you're aiming at with your left hand.
www.altay.vilko.net/dl/apache.JPG