Here's how I write stories:
A basic premise comes to mind.
Something like: "OOh, I like Gallagher girls, I like girl spies, I like girls doing awesome important stuff, I also like writing about dynamics between high school girls, and I like writing rich people-- HEY Elite."
Then I come up with characters.
Then I worldbuild.
Then I chat with people 'till I have a plot.
Then I kick my ass plotting and have to keep fixing it over and over again.
(of course, I do this by hand. it's easier to cross stuff out and draw cool diagrams)
Then, I start writing!