Start by placing your thoughts in a dreary, dark, horrible place. No electric, no plumbing, no way of getting gasoline for whatever vehicles may still exist, no form of heating apart from natural (think of chopping wood). Homes with windows blown out, walls ripped off. Rubble everywhere.
Imagine all these things and then place them on your sheet of paper (or in the computer).
Try to decide what sort of rule there is. Is it a group of roving and marauding gangs or a singular tyrannical leader?
How will you acquire provisions (food, clothing)?
The thought process for creating this is all within you, inside the crevices of your brain (unless of course the apocalyptic world is being ruled by zombies, in which case part of the brain may have become a midnight snack).
Best of luck and let us know how it turns out.