If they aren't rusted up already. Every bmw I've had the hubs have been rusty {just surface} so prep using a wire brush and some birtex or similar then some 500ish grit sand paper then clean with some white spirit
as above, on my previous car I didn't mask off-a swift drive round the block allows the pads to scrape off any paint on the swept area.
This reminds me that mine need doing asap
Rob