Breathing wise I would take short and small intake through the nose, and long unforced out breaths through a small gap in the mouth.
To give you an idea of the rhythm, consider a point moving along a sign wave:
1. Quarter cycle breath in. Don't breath in too much as you can just a small amount.
2. Quarter cycle pause, try to do this in an unstrained way and don't purse the lip or even close just don't breathe out.
3. Half cycle breathing out a small gap in the mouth, don't be too force full keep it it slow.
The deep breath idea is traditional, but there no reason to assume it is better, there was a study that found deep breath wasn't any better then short breath in is more the speed and volume that matters. It is ok to breath from you belly. Your diaphragm is used anyway, but it means you your belly is not tight an constrained as if you didn't move it.
If you have a tendency to hyperventilate during panic attacks, you want o breath less an slow down you breathing. I used to treat lockjaw like a good thing once I educated myself.
I have a friend on the sepctrum and when he melts down his breathing can be fast. One of the things that helps is putting his hand in front of his mouth so he can feel his breath. I think that is a really cool idea, as not only doe sit help regulate the breathing, it is also a sensation that takes you mind of what you are thinking.