OK here are some instructions on how to change the skirts on an R56. The process is almost identical for R50 and R53.
Step 1
Assemble the new skirt if this was not all ready done by supplying dealer.
Skirts as they arrived
http://i74.photobucket.com/albums/i2...2/PICT1700.jpg
Step 2
Undo the plastic screws under the car existing skirt. (jacking the car makes access easier). Remove the metal screw at the rear end of the skirt, im pretty sure it was a T10 or T15 screw head.
http://i74.photobucket.com/albums/i2...2/PICT1701.jpg
Step 3
Pull back door seal
http://i74.photobucket.com/albums/i2...2/PICT1702.jpg
Step 4
Dont be afraid here when it sounds like you are destroying the car. You have to be a bit rough. The clips that hold the skirt on are designed to break. Grab the rear end of the skirt and start pulling it away from the car.
http://i74.photobucket.com/albums/i2...2/PICT1703.jpg
You then need to lift the skirt up and away from the car. This will release the clips which are under the chrome finishing strip. Easier if 2 persons do this.
Step 5
Remove the broken clips from the cars body
http://i74.photobucket.com/albums/i2...2/PICT1704.jpg
Step 6
put the new clips onto the new skirt paying attention as to which colour of clip goes where and which way round they are facing.
http://i74.photobucket.com/albums/i2...2/PICT1705.jpg
Step 7
offer the skirt up to the car and start pushing the skirt onto the car from the rear working towards the front.
Step 8
Put the new clips onto the top surface of the skirt
http://i74.photobucket.com/albums/i2...2/PICT1706.jpg
Step 9
Put the screws back on underneath the car
Step 10
Put on finishing strip and re-sit door seal
http://i74.photobucket.com/albums/i2...2/PICT1707.jpg
Before pic
http://i74.photobucket.com/albums/i2...2/PICT1699.jpg
After pic
http://i74.photobucket.com/albums/i2...2/PICT1712.jpg
Whole job takes about 1 hour.