Just did this. Took under an hour. Really easy, don't be afraid of doing it. I went through the top. Mine was a 2009 328xi.
Only thing I would add is:
1. Keep the orientation of the boot correct and pay attention to how far up it is on your old knob to keep it at the same height. I left it on while removing the old one, just flip it up and over the top of the shifting stalk, it will hold it.
2. No need for string or glue. I bought the C clip from tischer, it was the same model. I knocked the old one off easily, heard it hit the ground. Old one slid on just as easy and is tight.
3. Angled so the V in the shifter has the point towards the rear. The other way was off.