• Lie on your back on a flat surface (or in bed) with your knees bent. You can use a pillow under your head and your knees for support, if that’s more comfortable.
• Place one hand on your upper chest and the other on your belly, just below your rib cage.
• Breathe in slowly through your nose, letting the air in deeply, towards your lower belly. The hand on your chest should remain still, while the one on your belly should rise.
• Tighten your abdominal muscles and let them fall inward as you exhale through pursed lips. The hand on your belly should move down to its original position.

https://www.health.harvard.edu/lung-health-and-disease/learning-diaphragmatic-breathing