I just picked up an E91 for the wife and wanted to do this mod as well, but the instructions for the steptronic with iDrive just didn't make sense and had me worried about damaging something. Luckily I got an E90 step loaner today, so what better way to figure out how this really works.
I haven't done the actual mod yet, but I did tear the center console trim apart and it's very easy.
I've added a few arrows to TambourineMan's pic. (hope he doesn't mind)
The leather boot and plastic cover are all one piece and held into the trim at 6 points. The 3 important ones have red arrows pointing to them in the pic above. You basically grab the front of the black plastic surround with your finger tips and press in towards the shifter to release that clip on the top. Then move to the front right corner and do the same. Once that corner is out, the whole piece can easily be lifted out by pulling towards the front of the car. It's not as terrifying as it sounds. After you have that out you'll need to grab the lower left side of the center trim (closest to the drivers seat) with your fingers and pull up to release one of the clips. Then just work your way around. Not nearly as bad as I was expecting. Just thought this info might help some step owners since I know none of this made sense to me. Obviously having the shifter in D allows you to slide the center trim piece over the shift cover for easier removal.
