One of the easiest ways to teach patients to breath with the diaphragm is to get the patient to inflate the posterior ribs. Many people simply breath up with a shrugging motion, never really getting rib expansion.
One easy drill I've been employing recently is to hook the shoulder blade so that my fingers are underneath the shoulder blade.
I tell them to inhale through the nose and push my fingers out from behind the shoulder blade by getting the ribs to expand. Usually after a few tries, they feel the ribs start to push against my fingers and it becomes easier to do.
Breathing 22,000x a day is an important task to get correct. Getting the patient to feel it correctly is very important. Give it a try and let me know what you think.