Its a doddle mr wilks.... Cost me about 30£ but i already had some polishing mops and wet and dry paper.. the real expense was the nitromos stripper, rattle can varnish, varnish remover pad for the drill and the m8 spline tool to split them prob about 50 £ if you dont have the other bits.
this was a quick post i put.
http://www.z4-forum.com/forum/viewtopic.php?f=2&t=38407&p=529756&hilit=+108%2A#p529756
this is the how to in the wheels section.
http://www.z4-forum.com/forum/viewtopic.php?f=28&t=15605
there's this one too another good guide
http://www.z4-forum.com/forum/viewtopic.php?f=28&t=10316
Just takes a bit of time.