Here’s how: Write something. Do it now. Now look at it not as an author, but as a reader or audience and ask questions about it. For example, I write, "It was dawn in the small western town." Now I ask:
1. What time of year was it?
2. What state?
3. Is it a ghost town?
4. How many people live there?
5. Is everything all right in the town?
6. What year is it?
Then let your Muse come up with as many answers for each question as possible.
Example 6 from above: What year is it?
B. Present Day
D. After the apocalypse.
Example D from above: After the Apocalypse.
1. What kind of apocalypse?
2. How many people died?
3. How long ago was the disaster, and so on.
By alternating between critical analysis and creative Musings, you will quickly work out details about your story's world, who's in it, what happens to them and what it all means.