All you need is some thin strong (hopefully new and not fraying) rope About 1metre/1.5 metres in length(depending on your height. I'm quite tall (6 foot 3) an the string folded in half should reach to about your hip with your arm fully extended horizontally). So get an appropraitely suEd price of rope/string. Then get an old shoe or leather or a rag. A shoe is actually pretty good. Simply remove the tongueand use the tongue. A price of leather is a bit harder. Cut a wide rectangle in it. A good size would be approx 7cm wide 20cm long. U need to punch 2 holes IN THE MIDDLE of the ends about 1cm from the edge. If the rectangle is longside nearest to your body then the left and right ends of the leather should be punctured (the short end). Then cut your string with one peice about 2 cm longer. This peice will wrap around your finger. Tie it to the leather/tongue with a solid knot. There are several knots u can use and half are a bit tricky to explain here. So just tie it and make sure it won't come out. Tie the other string to the other side. Now the sling is ready. Grab a rock or pebble or nut or something roundish off the ground and put it in the sling. Pick up the long end and tie it loosely to your index finger of the hand u use (aka right handed tie to their right) in the first joint from the top. Loose enough to spin without getting caught or snagged. Simply hold the other end in your thumb and index. Then spin the sling fast (gotta keep the ammo in the sling) clockwise on the side of your body. For a right handed person it should spin up toward the face then downwards toward your heel. When you reach a good speed (when it starts whirring) release the string between your thumb and index when the sling is near your heel on it's way up. Your Ammo will shoot out of the sling and go a good 50 metres if the wind is aiding you. Still hits pretty hard and hurts like a bitch at close range. And never intentionally I'm for te head. Especially with a honky nut coz again, it stings like a bitch.
You shouldn't shoot these things at your friends or foes (unless they shoot first) fun things to try with them is set up a wide target against a wall and try hitting it as accurately as possible. Alternatly you can have a competition with your friends seeing who can hit a beer can(s) first. Or go for a distance shot. Aim high and keep the wind pushing t your back.(if there is a wind). If your rope/string is getting caught on your finger try loosening it a bit more or get somethig with a lump in the end and tie it to that so that when it spins it can't slip off.
Do be careful coz these things were used as weapons in the past by a lot of cultures. and ideas about weaponry never catch on if they aren't effective.