Breathe. Just breathe. Not particularly deep, not particularly shallow. Just breathe. Breathe through the nostrils if you can, through the mouth if you can’t. Whatever you do, keep breathing. Just breathing, and be aware of your breathing.
Focus all your attention on your breathing. Sense the air upon your lips, or better, feel it touch the nostrils. Be aware of its warmth or coolness. Attend intently to normal, ordinary, everyday breathing.
Do not follow the breaths inwards. Do not hold the breath. Do not exaggerate the breath. This is not yoga. Just breathe, and be aware of breathing.
The breathing may deepen a little at first and then become more shallow later. But just become aware of breathing.