This is tricky. A kind of relaxation and tension is needed.
If you relax too much the walls of the breathing muscles
collapse.

The abdominal muscles have a minimal approach to breathing.
If you attempt to press the abdominal muscles out, you will
interfere with relaxed breathing.

If you attempt to force them in and up, it will have the same effect.

The intercostal muscles react involuntarilly. When freedom from tension in that area is lessened, they are more flexible and
can do their job without interference.

Relaxation and flexibility of the jaw, lips and tongue have a
direct bearing on allowing the intecostal muscles the freedom
to breathe.

Having strong intake of air does not mean good breathing for
vocal technique. The right amount of air pressure through the vocal folds produce the optimal sound.

Dan tien breathing in T'ai Chi Chuan is opposite to natural breating and can be inhibiting.

There is a device called a "Breath Builder" which consists of a plastic cylinder, a ping-pong ball and rubber tubing which is
helpful.

The rib-cage must be up in order for the breath support to take place. At no time should the breath be forced and at no time
should the muscles in the jaw, lips, cheeks and tongue be
"set". They should remain fluid and flexible. Any setting of any muscle will inhibit the breathing.

Breath control is the result of working with the above, facial flexibility, correct vowel placement (another can of worms), freedom from rigid setting, involuntary and free breathing and when this is
allowed, breaths can be taken at will.

Panting exercises are helpful to stimulate the intercostal area.

Some of this expansion is in the back as well as the front.

The diaphragm itself has little to do with the breathing mechansim. This is often confused with the action of the intercostals.

A good vocal teacher can help you enormously with this process.

Frank