Pull your abdomen in.  Breathe to your lower back.  This is the bottom of your lungs.  You can even tense your rib and stomach muscles to get more air in.  Fill the bottom of your lungs which is in your lower back (check anatomy).  The back of the diaphragm slopes down (diaphragm largest muscle in the body).  Then when you sing, relax everything and keep the breath in the abdomen, the abdomen goes out!   Babies bring their abdomen in to breath before they push the abdomen out and cry out.  Elvis when performing in Las Vegas can be seen standing very upright and tensing his stomach when he breathes in.

The teaching of pushing your stomach out when you breathe is incorrect.  Doing this will make you sing through your nose.

Singing a verse is done with a smooth continuous breath at the same flow rate and pressure. Subtle variations of this rule will be necessary depending on the song or beat, but that is the general principal.  You can lean forward a little at a time as you sing out.  This is the technique opera singers use (appoyo).

PRACTICE —
Hold your breath as long as you can. Time yourself. Work up to holding your breath and not breathing for several minutes.

The Caruso Walk

Caruso would get exercise walking.   1.  He would inhale for ten steps completely filling his lungs.  2.  He would hold his breath for ten steps.  3.  He would exhale for ten steps totally emptying his lungs.  4.  He would  hold his breath empty for ten steps.  Then he would repeat … Caruso quote, “You have to feel the breath vibrate deep down in the body.”

Pavoratti would bring in his belly button when he breathed in.
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The voice does nothing, the breath does everything.