Number plate holder VNA

Sip Art no: 22007000.

Please explain how this plate holder is fitted, I have a VNA chassis and I got a pressed aluminium plate 6.5cm square.

If you fit the plate holder to the frame first then surely you will not be able to attach the plate because you are unable to access behind the holder to tighten the nuts, vice versa if you attach the plate to holder first. Are there special self tapper screws to use, normal self tappers would surely be too long? 

How many of item 75205500 would you recommend to use?

That's 16.5cm square and not 6.5cm square always selling myself short boom boom.