Going off topic right away…it seems that the U.S. is thinking about how to cope in the event of a nuclear attack against some of our cities. An all out attack by, say, Russia would be unsurvivable, because even if you lived through the attack itself, the following nuclear winter would be severe beyond comprehension, with sub-zero temperatures even at the equator, for a while. There is no reason to think that Russia would launch such an attack; the ensuing winter would be doubly unsurvivable for them.

But one or two bombs, or a dozen, would be another story. The fallout won’t kill you if you stay out of its way.
At least, you have a much better chance.

After the immediate blast of gamma rays, which can kill but that’s irrelevant because the blast and heat will do that anyhow at close range, your problem is how to survive the next few hours and days.

(1) Get out of the “sun”. Immediately. Duck behind a tree, a corner, anything. The fireball is chockablock with energy and you’ll be burned if your skin is exposed to its light.

(2) Get indoors. Promptly. Any structure offers some protection from what’s next: a rain of debris and dust, maybe with water too. Either way, the stuff is highly radioactive. Most of the radiation is coming from elements with a short half life: minutes, hours, days. Much of it has minimal penetrating power. Just don’t get it on your skin and you’ve avoided the worst of it. Wash it off if some does get on; change clothes.

(3) Secure your drinking water supply. Toilet tanks are good drinking water. If you had warning, even a few minutes, you filled your bathtub and some containers.

(4) Build a shelter inside your home. Stack a table with books, magazines, etc. Stack more around the sides. Put your bed under the table. Every inch of shielding matters, and if lead isn’t available, anything is better than nothing.

(5) Pay attention to sanitation. A run to the latrine, in a far corner of the house, is safe enough. Diarrhea can kill you just as easily as exotic causes.

(6) Sit tight. The clock is your friend. Elements with a half life of a week are 75% gone in two weeks. Elements with a half life of a day, effectively all gone in two weeks. You won’t starve to death even if you have no food whatever.

(7) Fend for yourself. There won’t be any doctors, any hospitals, any anything. Even if they’re up and running, they’re out of reach, because the fallout will have made all travel impossibly dangerous. And even if they were up and running and you could reach them, you’d be one more patient on a miles-long waiting list.

The kind of nuclear attack a North Korea can deliver to the U.S. is not a world-shattering blow. If most everybody outside the relatively small zone of total annihilation took the advice, casualties would be greatly reduced.