Go to your local theater bookstore (such as Samuel French, in Los Angeles), and buy a guide such as "How to Be a Working Actor: The Insider's Guide to Finding Jobs in Theater, Film, and Television." This and several other such books list the casting directors and agencies who deal specifically with extras.
Send your headshot and résumé with a brief cover letter to the casting directors and agencies.
Follow up with a phone call. Inquire about their interview days.
Attend an interview day, when the agency holds an open call for prospective extras. They will take an instant-developing photo of you, plus statistics about your height, weight, hair color and age. Fill out all necessary information, and behave in a polite and professional manner.
Be prepared to be called to show up on a movie set to work with only a day's notice if the casting director or agency accepts you. Know in advance how you can be free and/or get time off work to go.
Show up 15 minutes early for your "call time" (see glossary). Immediately find the staff person in charge of the extras. Fill out all forms and sign in.
Stay in the areas allotted for the extras; behave professionally, as you would on any important acting job.
Thank the staff member at the end of the day and request that you be brought back to work on the movie again.