Remove the number plate from the vehicle. You may need to spray an anti-seize lubricant to the screws if stuck due to rust formation.
If the screw heads are large, they will need to go through the plate before the frame is attached so that the frame goes partly over the top of the screw head. Insert the top edge of the plate into the clips on the THIN side (top) of the frame first, and then gently lift the clips on the wide side of the frame over the other side of the number plate. Make sure all clips are properly seated over the edges of the number plate. Reattach the plate to the vehicle.