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. Next, you'll want to fold the fabric of the cloth across itself diagonally to form a triangle. When worn as a sling, the "fat" middle portion of the triangle will support your arm and the thin corners of the triangle will form a comfortable neckband behind your head. If, for some reason, you find that the sling isn't comfortable while folded, you can alternatively cut the square diagonally to create the same shape. When wearing the sling, your arm will be held in contact with cloth which, if you're making your own sling from home materials, has probably not been disinfected. Thus, if your injured arm has any open wounds, it's extra-important to insure that all of your wounds are cleaned, dried, and carefully bandaged before you introduce them to the sling's material. Below is a rough guide for cleaning minor wounds - see How to Care for Minor Abrasions and Scratches for more information. If you have a serious injury or see a bone at the site of the injury, don't attempt to make yourself a sling - go to a hospital immediately.  First, wash any open wounds BUT do not use extremely cold or extremely hot water. And use slow flowing water. Not harsh. If you do anything mentioned not to, you will surely hurt your arm even more. Remove dirt or any other debris from the wound with a clean pair of tweezers if it is not rinsed out by water. Apply a bandage over the wound. Use a bandage that covers the wound completely with no adhesive portion touching the wound itself. If needed, you may want to place clean gauze between the bandage material and the wound. You also may need to have a splint, in which case you should apply it first before the sling. Do not directly touch the wound unless you have medical experience. Next, you'll want to take off any rings, bracelets, and/or armbands that you are wearing on the injured arm. If the injured arm swells as it heals, jewelry (particularly tight-fitting pieces) can constrict the flow of blood to the arm, causing pain and irritation and even becoming stuck. Position your injured arm at a 90 degree angle across your chest (horizontal to the floor). Use your other arm to slip the folded, triangular piece of fabric over the shoulder of your uninjured arm. Let the rest of the fabric hang so that it lies behind the injured arm with the "point" of the umbrella pointing roughly towards the hip that is on the same side of the body as the injured arm. Use your uninjured arm to grab the corner of the triangle that is pointing towards the floor and bring it up across your body, over the opposite shoulder as the other end of the fabric, and behind your neck. Do this gently, as the fabric should now be touching the injured arm and can jerk on the arm if pulled roughly. The length of the sling material should be such that the injured arm can hang comfortably at a roughly 90 degree angle. Your fingers should extend just far enough beyond the "cuff" of the sling that it's possible to use them for simple tasks like writing while the hand is still supported by the sling. If this is not the case, adjust the fit of the sling as needed. When you've found a comfortable length for your sling, tie a simple knot in the two ends of the sling material to secure the sling behind your neck. If you need to adjust the height at which your sling hangs, loosen this knot and tie a new one either further "up" or "down" the length of the fabric. Congratulations! Your new sling is ready to wear.  If this knot uncomfortably digs into your neck, slip a small pad or towel under it. Be sure to avoid the hair at the nape of your neck when tying your knot. If you accidentally tie some of your hair into the knot, it can be painfully yanked when you move your arm or walk. If you have a safety pin handy, pin the two edges of the sling material near your elbow together. This creates a "backstop" for your elbow to rest against. Without this backstop, it's possible for your arm to slip out the back of the sling as you move or for the sling material to bunch up near your wrist. A sling transfers the weight of your injured arm onto your upper back and neck. This added burden can cause back and neck strain - even if you don't experience serious strain, over time, you'll probably notice that your sling causes the area between your shoulder blades to become fatigued. Lessen this effect by maintaining proper, erect posture. See below for brief posture instructions:  When you stand while wearing your sling, keep your back straight and your shoulders in a pulled-back but relaxed position. Keep your chin up and avoid slouching. When you sit while wearing your sling, keep your back against the chair's backrest, if there is one. Keep your back straight and upright. Keep your head and chin up so that your neck stays straight. Keep your feet planted on the ground. Don't sink into a slump or slouch. If you can comfortably support your arm on the chair's armrest, you may do so. If at any point while wearing a sling, you experience severe back or neck pain, see a doctor. Avoid wearing a sling if you have a known neck or back ailment.
Find a suitably-sized square of cloth. Fold the cloth in half diagonally to make a triangle. Clean and dress any wounds before wearing the sling. Remove any jewelry from the injured arm. Slip one end of the cloth under your arm and the other over your shoulder. Bring the other end of the sling over your other shoulder. Tie the ends of the sling behind your neck. If desired, close the edge of the sling  with a safety pin. Maintain good posture while you wear your sling.