I'm not at all certain where I first heard about the sling - most likely the David and Goliath story. I remember doing a preliminary Google on the subject and finding only Jim Burdine's site which, while extremely impressive, wasn't very helpful to the unitiated. So, I set about knitting a roughly 5"x5" square of material and fixing cords to each corner. Configured this way, one had to control all four cords, which made it a bit awkward.
Anyhow, I took it to an empty park with some friends, who proved much better at it than I. However, I found slinging.org shortly after and have been impressing them ever since, both with the slings themselves and with distance.