Notes

1
Find a suitably-sized square of cloth. This method uses a square of fabric to replicate the functionality of a real sling. Depending on your height and size, the precise size of the cloth you'll need can vary. For most people, a square of fabric that's roughly 40 inches (1 meter) on each side will work well. Ideally, you'll want a fairly inelastic piece of fabric - stretchy fabric allows your arm to bend and move, which can aggravate your injury.
One easy way to get a 40 inch (101.6 cm) square piece of fabric is to cut an old pillowcase or bed sheet that you don't mind ruining to size with a pair of sharp scissors or a fabric knife. In a pinch, you can even use your bare hands to tear these things to the right size.
When it comes to your sling fabric, err on the side of using a fabric square that's too big, rather than too small. A sling that's too big can be tightened by adjusting the knot behind your head while wearing the sling, but there's no real way to make a sling looser than the length of its fabric allows.