Imrpovise.

You start out with an idea, you transform the idea in your head, the more you think about it the more ideas you get, the more ideas you get the more the story expands. You do this until you know alll the characters, the conflict between them, and what needs to be resolved in the end. Then you find a meaning to all of it. Don't bother to write any of this down, you will just throw it away. Now, after about a month of thinking, you start writing it down, scene for scene, line for line, improvising everything from what you've gathered over the ideas in your head. You lock yourself in a small room, you wait until it starts raining, you turn on some music for the tone, then you don't stop until everything works and everything is finished.