Get the story out of your head. Don't worry about grammar or details, just get it on paper. I create a summary of the chapters explaining how I want the story to proceed. Some times I get carried away and write more than I should but there are no rules at this point, just get it out. Consider this your first draft. Go back after you are done and write the details. This is where you make it look more like a story. This is your second draft. The third draft, read over what you wrote and make it clearer and add any ideas that you might have had before. If you are unsure, go over it once more but don't go over it too many times as it would possibly ruin your story. This should be it, only 3-4 drafts nothing more.