Use scissors to cut the webbing to an appropriate length and width to fit the damaged area of the backpack strap.
Sear the edges of the webbing with the flame from a lighter. This will stop the edges of the webbing from fraying, thus making the repair much stronger and hardy.
Put the webbing over the tear in your strap, completely covering the damaged area.
Thread the sewing needle with thick sewing thread and, starting from one corner of the webbing, stitch the strap to the webbing diagonally across its width. You should stitch the repair in a zigzag pattern across the webbing, tying off the thread when you reach the end of the webbing. Repeat the process starting from the opposite corner in order to add more strength to the repair.
Secure the repair by stitching around the edges of the webbing. Stitch across the bottom of the webbing, before tying off the thread at the other side. Then, stitch across the top of the webbing in the same manner before completing the repair by stitching down each side of the webbing.