Once you have settled on your topic, it's time to develop a story. You should start by creating a "project treatment," a description and assessment of your goals for the project. You should consider your potential target audience, and how your film will be distributed. Knowing your audience will make your documentary even more effective. Develop a list of people you would want to interview, footage you need to acquire or shoot, and any other elements you think you will need to tell your story.
There are several different styles of documentary storytelling. Following are some different modes identified by Bill Nichols in 1991:
Poetic: Stresses the mood, tone, and affect much more than displays of knowledge or acts of persuasion. It also opens up the possibility of alternative forms of presentation of knowledge as opposed to the straightforward transfer of information.
Expository: Form fragments of the historical world into a more rhetorical or argumentative frame, rather than in an observational fashion.
Observational: The filmmaker simply observes what happens in front of the camera without overt intervention. S/he acts as a witness to history.
Participatory: The filmmaker lives among a group of people who are the subject of the documentary to gain a corporeal or visceral feel for what life is like through their eyes.
Reflexive: The engagement is between the filmmaker and his or her audience, rather than between the filmmaker and his or her characters in the documentary. This style also deals with the problem of representing and portraying history in addition to the social or political points being presented in the film. It's not just about what gets represented, but how.
Performative: Like the poetic mode of documentary representation, the performative mode raises questions about what knowledge is. It considers other knowledge that helps in our understanding of the world beyond factual information alone.
As with everything else in life, all documentaries start as an idea. Most documentaries are inspired by political or social issues, history, or stories about the human spirit. As you consider a topic for your documentary, first consider your own personal sources of inspiration. Is it a person, specific event, cause, issue or a place? Generally when you write about something you already know a lot about, or are inspired by a special event or issue, your passion will shine through.