If you see the bright flash associated with a nuclear explosion:
Lay face down on the ground and put your thumbs in your ears and fingers over your eyes. Breathe through your teeth. Since you’re laying face down the shockwave will mostly pass over you. (If you’re standing up it can cause your lungs, eardrums, and other organs to explode.)

Once the shockwave passes over you, you need to find shelter immediately and stay sheltered under as much concrete and steel as possible for 48 hours—this is critical.