Lie relaxed on an exercise mat and place a hand or a light book on your stomach.
You can bend your legs slightly and your feet are flat on the floor.
Breathe deeply into your stomach - your hand or the book should visibly rise and your chest should rise shortly afterwards.
Then breathe out deeply so that all the air can escape and your stomach becomes flat again.
If you can do the exercise well lying down, you can also do it sitting up.