First, take a bite of cord, a bite is an open loop.
Repeat until three loops adorn both sides of your warrior fingers trained for battle.
From one side, tie a simple hitch over all the bites of cord.
Then do the same from the other side, so that the two hitches are back to back.
After tightening from both sides, and a bit of fanagaling, the knot (which is just a modified sheep shank) looks like this on one side,
And of course you need to do the same to the other stay. And just like that it's like you have a different sling. I tested this knot slinging, and it didn't come out, but that doesn't mean it won't, so check it before you sling. And as always make sure you are throwing soft projectiles or that you are totally alone.