You will need the rear wheels off and the fit by slackening the screws securing the wheel arch liner. Then they slide in and a screw from underside secures the bottom bit. Tighten wheel arch liner screws and they're on.
I fitted mine and wihin 2k the OS one was off. I pulled in for a petrol stop on the autobahn and noticed it hanging. The bottom screw had fell out and the arch screws had worked loose. I refitted it with loctite on the screws and araldite on the flap lugs.