Think of it as writing the narration for a story on audio cassette. Later you can plug in visuals to illustrate this oral story. Keep the narration short and directed to your audience. You can also write narration one scene at a time. If the narration doesn't quite fit one scene, try moving that narration to another scene.
Narration can help connect a scene to a theme of the video.
The viewer doesn't need that much help. Think about the commercials you studied. Sometimes you'll hear the narrator talking about benefits that aren't directly related to the visuals on the screen, but the narration works anyway. Write narration as if you were speaking to just one person, rather than how might you think narration should be written. Write short simple sentences.