I second the recommendation to use the included self tapping screws in place of the orginal bolts on the rears. I spent 2 hrs trying to get the original bolts back into place before giving up (OK -- this included a trip to HW store to search for longer bolts). Possibly I could have done it, had I removed the rear wheels -- but even so I think I could have only gotten the necessary leverage if the car was fairly high off the ground.
I now think that this is the intended installation. Why else would BMW have included so many self-tapping screws with the flaps? (I still have 1 screw left over, even after replacing the 4 bolts with screws).
On the fronts, I kept the original plastic screws, rather than using the included plastic push pins as these seems much easier to re-install. They use a T25 screw driver.
One other thing that came in useful (if you don't remove the rear wheels) is a really short Phillips screw driver. I used a phillips bit for a socket wrench for this.