There are holes in the top left and right corners of the number plate, as well as the bottom. You will need about 3 ties, same as you would find on a garbage bag, about 6-8 inches.
Position the number plate at the center of the handlebard. Take one of the ties, put it through the top hole, and wrap around the handlebar, then twist it to tighten. Do the same with the other top hole.
For the bottom hole, pinch the cables together and wrap the tie around your cables. Do not put the number plate behind the cables or else it can lead to problems.