It depends on what/how you're using the shank. In Arabian halter showing it can be done 'live' or 'dead or captive', meaning when you shank (pull on the lead) if it's live it tightens and pinches. Captive means it won't tighten but can be used with a sideways jerk to pop the horse on the jaw to get the desired response.
If you're hooking to a barn halter for control of an unruly horse, I hook as one of the previous posters described, hook to the upper off side ring, down through the off side lower ring and either under the chin or over the nose and through the near side ring. Since I show halter, I NEVER put the chain over the nose as that will get a horse to drop his head, not desirable in the halter ring.
I have really strong feelings about using the chains at all and my stallions only wear them 2 times, either for showing halter or for being bred. Any other horse who is unruly on the ground gets re-schooled to get them some manners and I will use the chain to get through if necessary but only long enough to teach them how I want them to behave. If it's not a stallion, I don't use a chain except in the show ring, and I only use those 'dead' or 'captive' regardles of whether it's a stallion, mare or gelding.
I have told owners they'll have to move their horse if it can't be handled without a chain, I feel that strongly about it.