Pursed-lip breathing helps slow your breathing and increases the amount of air that you breathe out, which can help decrease the feeling of breathlessness. Breathe in slowly and gently purse your lips while you breathe out. Do not force the air out.
Try breathing out while holding a tissue in front of your mouth and have the tissue move slightly to provide a helpful visual. Start by practicing pursed-lip breathing at rest. Then, progress to doing pursed-lip breathing while you're standing and doing activities such as walking.