There should be priests, some kind of fire, jackel headedness, chanting, and possibly the use of grave matter (do your ghouls eat the bones too? If not then they could be used). Perhaps there used to be somewhere safe where the ghouls went to make more ghouls, but that's become more and more dangerous in recent times (sorry, yet more terrible monsters invade. It's an affliction of my mind), so they've had increaisingly to take to changlings in order to keep their numbers up.

I read an interesting short story a while ago in a wonderfully camp book about vampires where a house of vampires had a cellar full of ghouls. The ghouls rent the cellar so that they can have somewhere for the older ghouls to tach the younger ones the finer points of anatomy and surgery (and probably cookery). Are your ghouls surgeons, with their preciously long slender fingers? There could be a surgical element to the process of changing a human to a ghoul, mixed in with all the magis, and that could be qhy it doesn't always work?

OK, now my brain hurts :P