Any advice on running a session with a Big Magic scenario?

I think in my next session I might give my hunters an opportunity to do some big magic. My idea is there's an NPC who is a young kid with powers that they don't know how to control. The hunters can preform a big magic spell to help her create like an arcane focus to control her powers. The kid is being hunted by a group of vampires who want to drain her for her magic abilities, so that adds an element of danger. I've haven't ran a session with big magic yet. Any advice? what works what doesn't?