Step 0. Have enough food and water to survive at least a week without going outside.
Step 1. Don't die in the blast.
Step 2. Don't be outside when the fallout lands. Be underground or in the interior space of a large building.
Step 3. Avoid the fallout.
Step 4. Be useful enough to what's left of society that it's worth the effort to keep you alive.
Step 5. Pray that the combattants aren't vengeful and launch second and tertiary strikes.