Plan the amount of time you need for pre-production. Pre-production involves refining the perspective your documentary will be taking, performing research to compliment and enrich the film, contacting people of interest to arrange meetings and interviews, and planning locations where interviews and external footage will be filmed. Each of these tasks should be assigned a set amount of time. However, the amount of time you designate for each activity will depend on your particular project; to assess how long you'll need, consider having an initial planning meeting to set timetables.
Plan the production schedule. You should designate the amount of time you will need for the following activities: shooting all video and interviews, capturing audio (if required), and taking photographs which will be used for illustrating the film.
Plan the post-production schedule. This involves transcribing all the interviews, uploading video footage onto computers for editing, writing the documentary's narrative (the story which links the footage together), and writing the script. Finally, schedule how long you plan to spend editing the documentary into a finished product.
Create a document which contains all this information. This can be accomplished by making a table with a word processing program and listing the estimated start and finish time for each activity. Update the table as necessary if the schedule changes.