NOTE: I am not responsible for any damages, injuries, etc that are a result of sling construction or use.
Step 1: Materials
For this instructable, several things are required. A material for the pouch, cordage for the lines, leather scraps for a handle, something to cut everything, and something to make holes.
Pouch: It can be anything, really. In my opinion, leather is the best, or perhaps a woven pouch, which is not covered in the following directions. Any cloth will work, with varying degrees of durability, leather being more so than most types of cloth. I have cut leather from old shoes and an old backpack; I also get various bits and pieces from a local shoe store (the kind that makes and repairs shoes, not a shoe retail store). Leather can also be obtained at thrift stores in the form of old coats, bags, etc.
Lines: Simply because it is on hand, I use hemp. More specifically, I use braided hemp, which is braided and not twisted because a braid tends to have lower elasticity than a twist. Like the pouch, the material used for the lines can be really any type of cord. Hemp, linen, cotton, nylon, leather, wool, other natural fibers, other man made fibers, gut, etc. Realize that some materials will have more of a propensity to stretch under strain than others.
Handle: Scraps, roughly oblong, which can be made of anything.
Cutting utensils: Self-explanatory. Scissors, knife, etc.
Hole punching: A paper hole puncher will work on some leather. A leather punch works well. Scissors or a knife can make holes, albeit with more trouble and less tidily. Use your imagination.