The harsh truth of nuclear war in our modern era is not that everyone would die right away but that hundreds of millions would die in the combatant states while hundreds of millions would have to survive radiation and starvation. If you are one who sees the flash, has time to Duck and Cover, and for whom the light is distant you are not in the clear. Instead, soon fallout will be upon you a deadly black rain, and black ash will be upon you. How to survive this if you have not prepared at all up to this point? Dirt and Distance are your friends.
Your first friend is distance. Even with the largest bomb deployed by Russia If you are 5-6 miles from the hypocenter of the detonation and you took cover you will have survived, perhaps with cuts from flying debris, maybe some first degree burns.
After the fall out comes distance will also protect from radiation. The fall out will settle out into a layer on the ground and radiate gamma particles as it decays. The farther you are from it the better off you are. The best protection is to not be near where the fall out concentrates. 100 miles or more from the attacks and not downwind of the attacks. If you are anywhere near an attack, say in a city, being in a high rise building, 8-10 stories or more and on the middle floors in an inner room are the safest.
Dirt is your friend. Being underground is the best option for most people. The further you are underground the better. At least three feet of packed earth, concrete, or some combination of dense material and dead space will cut down your exposure to radiation by a factor of 10 - 1000. The thing about dirt as your protection is you need to be sure you are in a sturdy structure that will stand up. Again, if you are in a building with broken windows and still standing after the blast wave has passed head for the basement.
As for resources, food, water, an emergency radio. These are supplies FEMA has advised all Americans to have on hand for any emergency. OF all of this WATER is most crucial. In most buildings in the US we have hot and cold water on tap. That hot water comes from a water heater with 20 -40 or more gallons stored. Having the tools on hand to access it is crucial. As for food, do not eat anything, not in a sealed or covered container. Foraged, or hunted food should only be a last resort. Eating any meat of any animal that has died from radiation is asking for a worse sickness and death than starvation.
Dirt and Distance are your friends. Get as much distance and dirt between you and the radiation as possible. Water is the most important consumable.
A horrible topic to write about but we live in times that are horrible.