It really is quite tricky. One of VK's threads on the subject is here: http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=1499769, and his tutorial is here: http://www.atlantis23.com/erc_download.html. Although the links don't work any more since the forum software was upgraded beyond repair, Helgard's post in this thread: http://www.renderosity.com/mod/forumpro/showthread.php?message_id=2011134 outlines some simpler choices.

Apologies for the texual links, but the forking forum editor has stopped doing the link thing, and I can't seem to type in HTML as I used to either. Just imagine, if you will, how much better the forum would work if VK had coded it. :)