So, you've got your story, now what? Old fashioned as it may seem, try to get all the elements of your story written down in simple outline form using 3x5 index cards. Keep it loose, put each element on one 3x5 card so you can shuffled and re-shuffled them. Lay your story out and look at it. Examine all your possible elements. (Of course, you can do this with a computer too, but the index cards work well for sorting out thoughts and ideas.)
If you have old 8mm film from your youth, log it and list it as an element. Do you have old photos or access to old newspaper articles? Who are the people you want to interview and what subject mater will they cover? Record every element and every topic on a card and separate the cards with only one topic or element per card. Lay them out in an order that makes sense to you and use this to create your first outline. Keep these cards! You will use them over and over again.
Every story needs three things, a beginning, middle, and end. You must define where these points exist in your story. Does your story have a great hook that will involve the audience from the outset and hold them? Is it most effective when told chronologically or should it jump around in time? Will your story be narrated, will you write the narration, or will the subjects you interview tell the entire story in their own words? Will it be a combination? You must discover what is most dramatic and engaging about your story and tell it in a way that highlights those points.
How do you want your story heard? Do you want to create a formal documentary with voice-over narration and drops to interviews and B-roll, or do you want to do a Cinéma Vérité piece where the camera seems to just exist as it captures everything around it? Many documentaries these days have the raw reality look of the "Cops" TV show with hand-held cameras loosely carried on shoulders. Other documentaries use guerilla tactics; they surprise people by simply shoving a microphone in their face. Michael Moore is famous for this.
Regardless of your choice of treatment or subject matter, almost every documentary needs an emotional center. The audience needs someone or a group of "someones" to care about. A message or idea is not enough. The characters in your documentary will carry your plotline as strongly as your storyline. Very few documentaries based solely on intellectualism succeed. Give your documentary some heart and some emotion. Give us someone to root for.
As soon as you have determined the structure and treatment of your documentary, you are ready to take your outline and create a projected timeline and budget. In order to create a budget you must decide the format in which you want to shoot your project. Will you shoot film or video? What type? How often will you need sound? Will you be lighting with instruments or will you be shooting in available light? How many days and in how many locations will you need to shoot? How big of a crew and how much equipment will you need? How long and with what means will you edit?
After you answer these questions, you will be in the best position to get close to a bid for creating your project.


Documentary Storytelling, 2nd Edition - Making Stronger and More Dramatic Nonfiction Films
Videomaker's Documentary eBook
Documentary Film Making
Producing a Documentary Part 2
Documentaries Then and Now
How 3 Types of the Documentary Genre Are Made
The History of Making Documentaries
Documentary Storytelling
Documentary Making Tips - Character Development
Access Challenges for Documentary Makers