Each skateboard backpack will have 2 pieces of string fastened on the surface of the backpack.
When you want to wear a skateboard on a backpack, open the two straps, place the skateboard along the body of the backpack. Then, use your hands to attach the straps, and pull the straps so that it fits snugly, not too loose. Note that, put one end of the wheel on the straps to make sure it does not fall off.