Sit in comfortable pose with back straight.
Close eyes, wait till you are at ease.
Inhale and let belly expand naturally, then exhale by forcefully contracting the abdominal muscles.
Let the next inhalation happen naturally, and then exhale forcefully.
No strain. Absolutely no strain. Keep it light. Feel like you are pushing your breath out through the top of your head. A breath chimney.
Complete 10 rounds , one after another. Natural inhalation followed by forceful exhalation. Using the stomach muscles. Just clenching then back towards the spine a bit, suddenly, quick, light, easy. Like a controlled sneeze.
The chest is not moving. It is all in the belly.
Rest after 10 rounds and then repeat the whole thing 2 or 3 more times (2x10, 3x10, and so on).
Keep breath rhythmic, like a steam train, a cute quiet one, not a roarer.
Stomach should be empty. If you experience hyperventilation, desist from the practice. Don't do it if you have heart disease, high blood pressure, or hernia.