Fairly simple.
Remove the wheels of course
2-3 screws inside lower front attached to undertray
3-4 screws at the inside edge of the bumper cover
3-4 screws in the upper overhead area to frame
2 screws under the car along side the side skirt
You will need to pull the liner away from behind the fender lip and pull fairly hard downward. Remember to tuck the liner back inside the fender lip before attaching screws upon reassembly
Chances are you will need some body nuts for reattaching the bumper cover screws as they break easily
http://www.realoem.com/bmw/photos/07129904150