The easiest to follow tutorial I've found is here. Take the site's advice and use Control C for copying and Control V for pasting--It truly is easier!
You will have to go into your HTML (In the "Layout" tab click HTML), so be sure to preview your changes BEFORE you save them!
The tutorial ends with instructions to remove the nasty little blogger bar at the top, too. If you feel like your margins are too cramped, go ahead and follow the link for a instructions on adjusting the margins.
When you adjust your blog to 3 columns you will need to change your custom background to fit. I found that each of the free sites offered both a 2 and 3 column version of each design.
You may want to increase your banner/header to 940 pixels wide, too. You don't have to do this, but if you'd like it to take up the full space you'll need to change it