When singing, as it happens normally, breathing consists of two phases: inhalation and exhalation.
INHALATION
Stay in the erect posture, hold your head high, feel good support on your legs and stay relaxed.
Inhale with the nose and bring the air down, without lifting the shoulders. First, fill your sides and abdomen, then gradually go up and fill your lungs (as if you were filling a glass with water). Create an air column, as in fig. 1/1A (from A to B).
The diaphragm goes down pushing the abdomen forward, the sides broaden and the rib cage widens.
If you lay in the bed, you can feel it very clearly.