Well, it’s fairly simple. Get something going quickly in a circle and then release it when you want it to go straight. You can practice yourself with something on a long string. Whip it around your head and then learn when to release it to make it go straight in the direction you want it to.
Remember, however, that slings only do 1d4 damage unless you’re using a special bullet with additional damage of its own.
The technique is NOT to spin it round and round. You basically use a motion similar to a baseball pitcher, allowing the sling to amplify the motion. The projectile whips around at the back, and ends up traveling the same direction as the hand moves forward, but faster, so it basically releases itself.
Imagine a trebuchet with your arm replacing the lever. I think it probable that the sling was the inspiration for the trebuchet.
I’ve used one as well. Unlike in the cartoons where the sling is spun over one’s head like a propeller and then released, the proper motion is to swing it once around your head and then step into it while releasing the load somewhat like it’s being thrown.
The sling I used was about 5 feet long and it was very powerful. Easily lethal. It’s amazing how fast and how far one can sling a large rock.
Accuracy, of course, was non-existent and it would take years of practice to use the sling with consistent accuracy.
Thae last time I saw a sling in action was when my brother’s friend tried to demonstrate its power, and accidentally bashed out the long side window of my father’s 1959 Dodge station wagon. Did a fine job too; there were no pieces of glass larger than a hulled peanut.