The first link and the easiest way to do this was posted by mikem over at the bbtonuke boards.

A more advanced way to do it after following the above link was posted by wspearsall you can read about it here (I just mention this one because I have it setup on my site and really like giving the users the option of turning the left blocks on or off)

For more info your best bet is doing a search at bbtonuke.sourceforge.net for left and blocks.

Hope that helps!!

PS - Figured I would field this one for mikem because I know he is getting tired of pointing people to those posts over and over again