Read the film's script and determine the qualities that would make a suitable narrator. Watch a rough cut of the film, if one is available, to feel the atmosphere and mood of the movie. For example, if the movie is a documentary, the narration should be clear and objective, with correct pronunciation of any unusual words such as Latin names or scientific terms. However, if the movie is a drama or thriller, the narrator should have a more emotional or foreboding voice. If the movie is narrated by a character, the narration should be in the same voice as the character -- with obvious exceptions for age change, such as an older narrator describing past events in her life. The narrator's gender will also affect the mood of the film greatly; male voices make excellent narration for action movies, thrillers and some dramatic films, while female voices are more suited for romantic films and comedies. Once you have determined the qualities of a suitable narrator, select a narrator based on these criteria.
Set up an area to record the narration audio track. Choose a setting with minimal background noise and select a large enough room to avoid short-delay echos. Set up a microphone stand and connect the mic's output to the computer's input jack or a USB adapter. The narrator's position should be standing or sitting upright with chin kept out -- standing allows the narrator to get best use of his voice. The microphone should be 8 to 12 inches from the narrator's mouth to avoid audio clipping or "popping p's." A pair of headphones should be used to monitor the sound. Keep some water nearby at room temperature, allowing the narrator to hydrate his throat. When all of the equipment is connected, record a short demo to test the setup and check the sound quality. Make any necessary adjustments before recording the actual track.
Record the narration audio track using the recording area. Make sure the narrator or voice actor has a copy of the script, and discourage her from ad-libbing. One common technique is to record the narration while the video track is playing, allowing the narrator to sync her voice to the time frame in the movie. The other common technique is to write down the time cues ahead of time and record the narration independently of the video; this allows the narrator to speak in a more natural and conversational tone, although it sometimes requires the video editor to edit and clip video footage to synchronize the timing. This is especially true of animation, in which the audio tracks are almost always recorded first so the animations can be synchronized. Perform retakes after any mistakes or where the editor or narrator is unsure of the most fitting way to speak a line. Record 10 seconds of silence to capture any ambient noise in the recording environment; low levels of ambient noise can later be filtered out with an editing program.
Edit the audio track in an editing program such as Audacity, WaveSurfer or SoX. First use a ambience filter or the software's NR (noise reduction) feature to remove any background noise that may have been recorded; use the 10-second clip of recorded "silence" with the filter to establish the baseline of ambient noise. Use audio compression to even out the volume dynamics, and use normalization to bring up any quiet parts if needed. Adding some level of reverb can help give the narration better tonal quality, as well as giving the track a different acoustic sound that helps viewers distinguish the disembodied "voice-over" from other dialogue. Add any other desired effects, such as voice deepening, bass enhancement or voice masking.
Import the audio track or clips into the video-editing software. Using the editor's timeline, drag the clips into place and synchronize them with the video. Watch each scene with narration a few times through and adjust the timing cues for the audio to better sync the track if needed. It is often necessary to have the movie's background audio taper off to a lower volume during narration to give the narration more focus and reduce noise. Also taper the volume of the narration tracks at the beginning and end so that any ambient noise remaining in the recording will be introduced and fade away gradually. This greatly reduces the "choppy" quality of voice-over narration. Save your work when you have finished incorporating all of the narration audio.