Very easy. I did it for the first time recently, with a few simple instructions from the forum. There's a thread somewhere with the details in.

 

Thanks for all the input - the shims will be staying!

 

The shims also help to stop the caliper piston from embedding itself into the rear of the pad. Haven't had the S long enough for this to be a problem for me, but I have had it in the past when I've left the shims out.