The lettering bit clicks in place...I think you just slide the number plate in the main surround then the lettering bit clicks in place over it to hold the number in place.
You have to screw the main bit to your car...the rear is easy...the front I think you have two choices,
1. remove the front bumper and pop out the existing number plate holder OR
2. more messily but easier just screw the new number plate holder on top. (I opted for this, the lazy way)
Depends how fussy you are really, others can advise you on the best way to do 1, as I couldnt be arsed with it.