Decide on the subject matter for the script. You can choose anything you are interested in, but try and focus on something very specific. Avoid choosing a vague topic such as "Facts about the Earth." The documentary must be short, so the more focused you can make your subject, the easier it will be to write.
Create a list of business, people and sources that you can interview and cite in your documentary. Brainstorm as many ideas as possible. Go through the list of ideas and eliminate any impractical options, such as flying to Zaire to interview the indigenous people, unless your budget and subject supports such a move.
Develop a style and mood for the documentary once you have determined the direction the documentary will take. If the documentary deals with a sad subject, choose music in a minor key, with a slow tempo. Happier subjects, such as babies being born, can use more upbeat or even silly music. Take some time and leave notes about the mood in the script. Specific song selections are helpful as guides.
Create an outline for the documentary once you have shot about half of the footage. You can't write a script until all the footage is available, but you can provide basic notes about the beginning, middle and end of the documentary. Create three sections: introduction, discovery of material and conclusion. Fill in those sections with notes with references to particular footage.
Write the script once the documentary outline has been written. Be prepared for changes in the script even after the script is written. With the outline created, fill in information about any additional footage needed for the script.
Shoot the remaining footage and then polish the script. The script will help you to organize your footage and ensure that the documentary flows logically and addresses important questions.