How do you plan a story?
I write the whole thing out first, whether it's a novel or a short story. Then I see who the main characters are, who should be a main character but isn't and which characters don't work.
Afterwards I edit chapter by chapter, never moving on until each chapter is right. Then I start writing it out again.
It may take a few rewrites, a few days, weeks or months, but I get there eventually.
Occasionally I even get the story published.