Honestly? Not very.

I grew up in the 1980's doing duck-and-cover drills in school until the Berlin Wall came down. Growing up a science nerd who practically lived at the library, it was pretty obvious that duck-and-cover would do jack-and-shit if anybody ever pressed the big red button. So, I had to come to terms with the possibility that somebody might nuke where I grew up, probably far too early than was good for my mental health.

What it comes down to is this: In the unlikely event that somebody pops off a nuclear weapon, there is precisely dick that you, I, or anybody else likely to read this thread could do about it. It's way above any of our pay grades, to steal a phrase. The only thing you can really do is take heart in the fact that there is nothing you did to cause it and nothing you can do to stop it, just like a solar flare or a gamma ray burst. The best you can hope for is that you're somewhere in the first two blast radii because the most likely outcome is a bright flash, and that'll be that. No suffering, no radiation sickness, no cancer eating you alive months to years later.

I get that's not the kind of response you're looking for, and I can't blame you for that. However, neither am I willing to lie. Nuclear war is possibly one of the most horrible things the human race is capable of, and there is no happy medium, nothing reassuring, nothing hopeful that can be said.

Hug your loved ones.