Truly effective breathing involves long slow exhalation and natural ( not forced or excessive) inhalation.
Lie on the floor in a supine position
Place your hands on your belly, palms down to let them be heavy.
Breathe deep into your belly as if the breath could meet the touch of your hands there.
Feel the warmth of your hands on your belly.
Feel the breath down in the core of your body. Feel it in your back. And in your sides.
Let yourself relax.