It looks like there are some rivets, clips, and bolts that need to be removed. The rivets are expanding rivets so they will probably need to be drilled out, then replaced with new rivets. You also might need to replace the clips. Check out the link below (pieces 7, 9, 10, 11):
http://www.realoem.com/bmw/enUS/show...diagId=51_7466
Doing that with the bumper off will always be easiest, but that's not to say that it cant be done with it on.
Good luck!