Determine the type of voice you will need for your production. Define your target audience and the mood you wish to impart from your film. From there, think about what type of voice will fit the purpose of your movie, whether it is male or female, a low or deep voice and other voice qualities you may want for your film.
Determine the budget you're willing to allot for a narrator. This will determine the type of voice talent you can hire for your movie. The more experienced and more talented narrators will naturally command a much higher price for their services.
Look for a suitable voice talent. The Internet is a great starting point. Many narrators have personal websites with samples of their work. Alternatively, you can get in touch with talent agencies so you can tap into their pool of talents. You may also get in touch with other people in the film or television industry and ask for suggestions regarding suitable voice talents. You may also post a search for voice talents on the Internet or on movie-related job listings.
Select a number of suitable voice talents and ask for samples of their work. Before listening to their work, think once again about the ideal voice your are looking for in your movie. Try listing the qualities of the voice you are looking for so that you do not forget them.
Listen to the samples given to you by the voice talents and choose which among them will be your ideal narrator. Compare their work with each other and with the ideal voice you are looking for. If the works you have listened to don't fit the type of voice you are looking for, then you can continue to look elsewhere for your narrator. Otherwise, you may instead choose the talent whose work is as close to what you are looking for as possible.