Is it your first time? If so, just ask your players. Ask questions like crazy.
At the first session, either just ask them "So, what is the apocalypse like?" or have them pick playbooks first and then start asking them about their characters. Hardholder, what does your hold look like? Oh, it's a derelict building in a ruined city. Chopper, what are you and your boys riding, and where? Oh, you're riding on a sprawl of high ways connecting the city ruins. So, what's the wilderness like? Desert, forest, tundra? What's dangerous about it?"
In play, ask things like "Okay driver, where do you never go no matter how well you get payed? Oooh, there's a nutjob cult living in an old crater, I see."
If your players are shy or inexperienced with this sort of play, ask small questions and follow it up with more questions. Like so:
"So, Angel, where do you get your supplies?"
"The market"
"Cool, where's that?"
"Um, it's in an old building, I guess"
"Yeah, what kind of building do you think?"
"An old football stadium"
"What does it look like now?"
"It's been rebuilt in sticks and blankets and other scrounge"
"Nice, and what does it specialize in? What's the best part of it? What's scary about it? What does it sound like, what does it smell like, who wants it demolished?"