Step One: Measure out your cord. You will need two arm lengths. The heavier and thicker the cord the longer your initial piece has to be.
Step Two: Tie two overhand knots in the middle of your cord. The knots should be spaced about 3 ½ inches from each other.
Step Three: Untwist the section of cord between the two overhand knots and split the strands into two separate but even sections. This will form your pouch.
Step Four: Pass one cord through the center of the pouch to retwist the strands to form the pouch.
Step Five: Using an overhand knot, make your finger loop and release knot.
If the finished sling has an unusual or unwanted twist to it, as long as you're using natural fibers you can wet the sling with water then hang it with a stone in the pouch. Allow it to dry and the sling will be find afterwards.
EDIT: This sling is similar in style to a sling from New Caledonia on disply at the Natural Museum, Australia.