Practice yogic breathing. It is a standard technique used as a gateway to several more challenging, life-changing breathing techniques.

Sit in a chair or cross-legged and upright on the floor and relax the shoulders.

Place one hand over the navel and slowly breathe into the belly a few times.
You should feel the belly expand with each breath in and deflate with each breath out.

Move the hand up a few inches so that it’s covering the bottom of the rib cage. Then focus the breath into the hand location for three to five breaths.
This should expand the ribs with each inhale and retract them with each exhale.

Move the hand to just below the collarbone and breathe deeply into this area for a few breaths.
Imagine the chest spreading out and withdrawing with each exhale.

Connect all these motions into one breath by inhaling into the stomach, the lower rib cage, and then the chest.

Exhale in the opposite direction by first emptying the chest, then the rib cage, and finally the stomach.
It may help to use a hand and feel each area as you breathe in and out of it.

Continue this same sequence for about a dozen rounds.
These motions will feel very awkward at first, but after a few breaths, they get easier.