If you have a look though my progress thread (link in my sig) you'll see my pics and the logs I did. You basically remove all the ridges from inside the box and sand it as smooth as you can. This means that there won't be as much resistance in the box for the air to pass over. I got an extra 3bhp and 7lbft from doing it on my S3.
The easiest way I found was to use an extremely sharp stanley knife to score the bottom of each plastic fin then use a pair of pliers to snap the fin off. This will leave you will some tiny little lips to sand down. If you have a dremmel and a sanding pad it's a lot quicker to do.