Found here. Short version: easy enough to do the robes-and-chanting-and-sacrifice sorts, but you can do better. Also, David Drake’s collection of short stories (Balefires) has a cracking good story in it (“…Than Curse the Darkness”) that should be required reading for anybody trying to do Cthulhu Mythos-style cultists. Drake absolutely sells what it would take to get people with the proper nihilism – and it’s alarming, because he actually toned down the historical record a little when doing it. Check it out.