You play yourself, with what you have on you right this moment, just as the world starts to fall apart.
You aren't the protagonist, you're the poor schmuck standing 3 feet to the left of the Brad Pitt. You're the person in the car, watching the protagonist with plot armour cinematically slide over the hood to escape the baddie. And you've got a few seconds to get your kid out of the child seat before the baddie stops chasing the protagonist and starts trying to crawl through the window.
Powered by the Apocalypse, the game engine created by Meg and Vincent Baker for their game Apocalypse World, but with a news mechanic for glimpsing bits and pieces of things falling apart elsewhere and the relationship mechanic removed.
There is no end game, no narrative arc. You are not the protagonist. This is a vignette of you, on the worst day of 7 billion people's lives.
At the beginning of the game, players roll a d20. The highest number is the primary player character (PPC). They pat themselves down, look at the room they're in and peek out the windows, and the game starts in that exact location. The game takes place in the streets and city of whoever rolled highest.
To prevent jarringly incorrect assumptions (assuming they live in a house rather than an apartment or such), the PPC can encounter the other players only outside their starting location. They play themselves, just transported to the PPC's city. When in doubt, try to make it as easy as possible for the player to be able to imagine themselves in the situation. For the PPC, they can just look around the room they're in. For the others, it'll be a lot easier to picture themselves in a 7/11 or something than standing right on a street they've never been on.
Different scenarios can of course be used, but the more elaborate and unusual they are, the more the player will have to extrapolate their character and the more the character will feel alien to them. Try to keep the scenarios more like "you're at the cafe you stop by on the way to work every morning" than "you and your friends win tickets for a cruise to the Bahamas." The simpler and more mundane the scenario, the less distance there will be between the player and their character.
Check the Dungeon World Guide and the Simple World guide for an introduction to the game mechanics of PBtA. There are also links to some good articles on GMing and playing in the GM sheet. I removed some components that seem awkward when used in a game where a player plays themselves, such as the relationship mechanic. I changed the Fronts and Threats mechanic to be more chaotic and less narrative driven.
There's also a BLIT module where instead of zombies, you are dealing with procedurally-generated images that shortcircuit the human visual cortex and cause fatal seizures and other neurological trauma.
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