For many writers, brainstorming is the most enjoyable part of the writing process. There are no rules at this point; the only purpose is to generate as many ideas as possible and to let your imagination run wild. Set your mind to the story at hand and allow it consider all possibilities, jotting them down on a piece of paper as they arise. Consider the introduction of characters, unexpected plot developments, and possible resolutions. Nothing is too outlandish in the brainstorming stage, because everything is going to be revised and edited in later stages. Don't worry about whether something makes sense or fits in with the rest of the story. Whatever comes to mind, write it down immediately.
The rough draft is where the real work starts, and where you must begin to be discriminating with your ideas. Go through your brainstorming notes and begin to construct them into an engaging and compelling story. It will quickly become apparent that many of your brainstorming ideas will not work in the story. This is fine and part of the process. Don't destroy them, keep these ideas in a separate file, either on paper or on your computer, they may be useful in another story. When you have winnowed down your ideas and plot development into a sensible story line, begin to flesh it out into a rough draft. As in the brainstorming stage, but in a somewhat more controlled manner, don't be too concerned about making everything perfect. The main thing at the rough draft stage is to continue to progress and not to get bogged down and give up. The purpose is to develop the main story line and to know how it's going to end before you go back for revision.
Revision is not where the fun of writing lies, but it is critical in the process of transforming a passable story into an excellent one. The revision stage is as much about reading as about writing. Read your story as another person would, trying to see it objectively. If possible, put it aside for a week or two and then go back to it and read it with fresh eyes. Edit anything that seems superfluous or boring. Keep all of your edits in a separate file. Do this not only so that you will have them for future reference, but also so that you will not be hesitant to edit things out of your story. Many writers cling to their creations and shrink away from cutting words; you are less likely to get into this habit if you know that the words are still available.
As the process of writing the story progresses, you will find yourself moving back and forth between these stages. If it turns out that an entire section is missing that is necessary for the plot, you will be returning to the rough draft stage or even the brainstorming stage to create this section. Remain flexible in your mind and allow yourself to mix, modify, and alter any stage or process to suit your needs. Stages of writing are a guideline for your success, not a set of unbreakable laws.