I've never seen a DIY... so IF you do this on your own... maybe you can take some pics and make a DIY for us 1Addicts.
On BMW's the serpentine belt is fairly easy to replace(although, I have not done it on my N54 just yet)... look at the idler with the shock/spring... pry off the plastic cap on the idler's face... then use a wrench on the exposed bolt to compress the tensioner, slip off the belt and install the new belt. That's how it was on my e36 and the Germans don't tend to change much over the years. Why reinvent the wheel - you know?