It's pretty easy. You copy and paste your conversation from just about any chat client and this wondiferous thing generates the HTML and the BB Code to make your conversation all pretty. All names are automatically bolded and colorized, to help keep people from getting bored and pissed off at your chatlog between LolMonkeyO and LolMonkey0 (which probably consists of crap anyway, if you're posting a long chat log.)
Try it out. I've only tested this in Trillian, being too lazy to access other chat clients, but I believe it will generally work for anything. Drop me a line if it doesn't.