I wish I could find my sons, but let me see if I can explain it . I would take a fabric that you like . Measure your arm from the elbow to the wrist to get the length and add about an inch for the seams. Make it about 5 inches high. Fold the fabric in half and sew it closed at the elbow end. Sew up the other end seam ( just fold over like regular sewing ) for the wrist area, but don't sew closed like the elbow area. Then for the shoulder strap area you could use the same fabric and make a strap, measure how long you need it to be comfortable for you,then take off about 3-4 inches, or you could buy the belting ( I think that is what it is called , I use it for straps on totes ) Also a D ring and some velco that folds up and attaches to itself. Attach the fabric for the shoulder strap to the sling with both ends. One on the front of the sling , the other on the back part of the sling. try it on , it should be shorter than you want. At this point figure out where you want to put the adjustable vleco. Cut the strap. Attach the D ring to the bottom part of the strap that you just cut. On the top part of the strap you just cut attach the velco that attaches to itself.
Have I totally confused you, :? It really is simpler than I probably made it sound