from memory I think you drill holes in the top of the door and maybe even in the vertical pannel. The guard fits into the top holes and then you use bolts (with platic thumb grips) that screw into the vertical pannel of the door. Basically the guard fits onto the metal frame inside the door, not the plastic cover.
Im sure I found a fitting guide on the web after some googling about once. I may have it, ill try and dig it out if I remember.