Write the script. The script should be written and revised, before production, during production, and during and after editing. In short, you're always massaging the script. The script is what will be read by the narrator. The script should sync up well with the film's images and background sounds and music.
Purchase equipment. A home PC is capable of handling the recording. However, use a professional microphone to achieve the highest recording quality. Purchase a professional microphone from any music recording equipment store. Prices range greatly but do not spend more than $500 on a microphone as those priced below it should be adequate. Select audio recording software. Cubase, Adobe Audition or Sony Soundforge all offer excellent recording and post-production tools. These software suites are available for trial download.
Set up the recording studio. The recording environment should be extremely quiet, with little background noise. Turn off all unnecessary noise makers, such as air conditioners. For added insulation place mattresses against the walls or tape egg cartons on the walls and ceiling. The oval surface of the carton deflects sound and enhances acoustics.
Record the audio. Team the narrator work with a technician who is familiar with the film and equipment. When recording the audio track, have the narrator watch the scene for which they are doing the narration. This helps add context for the narrator, who can adjust vocal style and tone based on the scene. For example, the narration "nature is amazing" should be read very differently for a scene of a waterfall or that of a lion attacking a zebra.
Practice. Do several takes for each scene and then edit the best one. Use the software to normalize the audio track and adjust levels. Export the audio to the film's soundtrack.