Creating a world

Or world building, take your pick. In my first published story, Inamorata, the era was Venice circa 1690. I knew the city, looked up a couple canal names on a map, doubled checked the fashion, and bam! World created.
Making one up from scratch is a lot harder.

I'm working on two separate and distinct shorts for Ravenous' Apocalypse Today: Love Among the Ruins. They both take place in the same world, but in different areas with different experiences and different outcomes.

I had no idea it would be this hard! Taking a slice of the past and adding in a bit of historical intrigue and a heavy dose of romance and sex is one thing. Translating that heavy dose of romance and sex into a world of my own creating? More difficult. Not because, I think, of the transition itself, because of the world itself.

Coruscant from the Star Wars films, check it out. A city-world. Pretty cool!

So my questions in creating this brave new world are:
WHY is it a post apocalypse world? WHAT happened? WHEN did it happen? WHERE did it happen? WHO did it happen to? WHO made it happen? HOW did it happen?

Who, what, where, when, why, and how? Journalistic questions for an erotic romance. It all works. And after much frustration, I'll be using those questions from now on, whether going forward with this hell-world, or (finally) finishing my Regency ménage.

How do other people world build? I'm curious.