Just a quick answer for now. Others may elaborate before I get a chance. Essentially its all about your angles to the light, and you should see an effect in the viewfinder as you rotate the front of the polariser if you have it right.
A bit of a quick hunt for a couple of links that may be helpful as a starter.
http://www.luminous-landscape.com/tu...larizers.shtml
http://www.offrench.net/photos/artic...ing_filter.php
Don