1. You must have an alignment that is Chaotic. In order to do that, you must never tell the truth and do the opposite of everything you are expected to do.
2. Talk to Barking Wilder, located near the Smoldering Corpse Bar.
3. When talking to him, pick the options where the words are out of order (i.e., moon/snoom), sing the rhyming song, etc.
4. Do NOT choose "Can I join the Chaosmen?"; choose the option that does not make sense.

Thanks to XaositectR for the information.