This is how I make a rock sling that is very powerful. It took about 8 months for me, my uncle, and my cousin to perfect this design. After lots of reading, testing, designing, building, gathering materials, and tying knots, I present to you this instructable.

Step 1: Materials/tools

you will need:
1) some leather (easily gathered from an old glove, a hobby/ craft store, or a cow)
2) Parachute cord
3) paper ( to make a template)

tools:
1)sharp scissors
2) a lighter
3) a printer connected to a computer( to make/ print template)
4) a hole punch( a leather hole punch with multiple sizes will make your building life much easier)
5) a permanent marker with a good point

Note: please familiarize your self with these two knots: perfection loop, and Ashley stopper knot.

Step 2: Template

To make the template, go to a drawing program on your computer and find the circle making tool, and place a circle on the screen. Then find the show rulers button and enable it. At this point you should be able to see inches marked on the sides of your screen. Adjust your circle to an oval shape that is 5 inches long and 3 inches wide using the rulers as a reference. Then add two 1/4 inch circles across from each other 1/4 of an inch from each long end edge. be sure to center each using the rulers on the screen. Your template should look like mine in the above picture. (don't forget to print it.)

Step 3: Cut to the Chase

If you successfully caught your neighbors cow and cut the hide off it...... jk, don't do that. Now you need to trace the template onto the leather( make sure you have tough leather, the weak stuff will tear and could be dangerous if it broke during use) and cut it out, also hole punch the template so you can trace the hole and punch it out on the leather.
Note: Now is the time to paint on your leather like I painted a dragon on one of mine( if you want).

Step 4: Knot What You Had in Mind

This step requires moderate knot tying skills. Remember those knots I told you to learn earlier? this is where we use them. Start by cutting two, four foot lengths of paracord.
On one side of one tie a perfection loop big enough for 3, fingers to fit through and with extra sticking off to the left for adjusting the loop size. On the second, tie an Ashley stopper knot on one end and leave around 2 inches sticking out the top and fray the end to make it soft.

Step 5: The Hard Part

What you need to do now is adjust the perfection loop to where (dominate hand) your pinky and ring ringer fit comfortably inside the loop. Make sure you still have extra sticking out the side of the knot. The purpose of this? let me explain... I have seen where some make a loop for the middle finger, this I find is not as comfortable as this way of holding the sling(see picture). The extra is for adjusting easily for larger/ smaller hands if you were to let a friend try your sling. Lastly you get more power because the loop isn't tugging the life out of your middle finger.
Once you have adjusted you loop now you need to measure the distance from your knuckles in a closed fist to the ground when you are standing relaxed. Once you have your measurement subtract 3 and 1/2 inches. This is how long the sling should be from the knot in the loop to the attachment point on your sling. This keeps the bottom of the sling pouch an inch off thee ground at all times, allowing you maximum power in your throw. But let me show you how to attach the pouch before you cut the paracord.

Step 6: Where Two Ends Meet

Now you need to attache your ends to the sling pouch. As you can see in the picture, you just feed thee end through the hole and tie a figure eight knot. pull it a little tight and ajust as needed to get that length measurement you got in the last step. For the release end, you tie it the same but it won't be the same length, so you will have to adjust the knot on the pouch to get it level when you hold the sling like you would when your about to throw.
Note: I prefer the fuzzy part to be the inside to get better grip on the stones.

Step 7: Throwing/ Ammo

I like to throw with an overhand Belauric style but the side hand and under hand are also quite effective at sending rocks to smash through a 5/8 inch advantech flooring board( when we got one through, it would keep going and hit a tree in the woods behind our board, and it would sound like some one took at sledge hammer to a tree as hard as they could) lol.
For ammo we have found(we as in my uncle, cousin, and I):
1)Tennis balls are great for practice, and epic games of catch.
2) Glow in the dark dog toy balls are great with a 1000 lumen flashlight at night for fun in the dark.
3) Lead 6oz. fishing sinkers do serious damage to wood boards.
4) Concrete molded in tennis balls explode into dust( yes, dust, I am not kidding) when slung at a big board.
5)Concrete molded in easter eggs work 3 or 4 times before the concrete is ruined.
6) Weird shaped rocks sound like super mad bees when they are thrown fast.
7) A bunch of small rocks tied in a plastic bag will make a shot gun effect because the bag tears apart and due to the spin pulling the rocks outwards.
8) Racquetballs are really bad for use as sling ammo..... would not try it again.
9)Water balloons are hard to sling, better when frozen.
10) Rocks work pretty great but they are all different, hard to bee consistent with them. Smooth, round, river stones are better.
11) Chicken eggs are fun to sling, they fly good too!
12)Golf balls work fine, but be careful, they can bounce!
13) Bouncy balls are weird....
14) You can get a big bad of river rocks in the gardening center of a hardware store for around $5 that will give you a whole day of slinging.( I think it was a 30lb bag)
We have tried tons of ammo and experimented lots. I hope you do too, just stay safe and use common sense.

Step 8: Safety

Let my start by saying that I am not responsible for anything YOU do with your sling. Please use you're sling at your own risk and do not sling at people or pets. Be sure to watch your surroundings to prevent accidents and check behind your target to makes sure nothing you don't want to break is behind it. The sling is reported to have a range of up to 500 meters, so if your going to sling then make sure you have a good backstop or a very long open field with no livestock or persons inhabiting it.
Don't forget to comment and share your thoughts, we would love to hear them!
Be safe and happy slinging!