Write clearly, concisely and matter-of-factly. Imagining that you are writing a television documentary can help you do this more effectively. The secret is to make your reader believe that the people and events in your story are real. Avoid using too many adjectives and elaborate descriptions. For example, instead of waxing poetic about a character's looks, introduce factual information about where she went to school, her geographical location and details about her daily life.
Adopt journalistic techniques. If your story is about a murder, you could write fictional newspaper articles about the event, transcripts of prison interviews with the killer, or newspaper interviews with the victim's family. You can use this technique even if your story is about fantastic events that are unlikely in real life.
Blend fact and fiction. Many authors who write in a documentary style base their story on real events. Daniel Defoe's novel "Journal of the Plague Year," for example, is a fictionalized account of the London plague of 1665, written in memoir form to give it added reality. If your story is set in the 1920s, you could include published newspaper accounts about flapper girls or the stock market crash. Even if your story isn't based on real events, you can make it seem so by adding factual information related to the geographical setting or time frame. Just introducing accurate information about the weather and local shops and schools or contemporary events gives your story an aura of authenticity.