Record a demo containing three of four songs demonstrating your vocal range and skills. Consider more than one genre to help showcase your flexibility. Studio singers need to be prepared to sing in a variety of styles and under a variety of conditions.
Update your resume often. Every time you do a studio job or any job related to your skills as a vocalist, be sure to include it in your resume. Consider making simple business cards to go along with your resume. In many instances, a producer or recording engineer may need a singer on the spur of the moment, and he is more likely to keep your business card handy than to go in search of a resume.
Visit recording studios and introduce yourself. Drop off your resume, business card and demo. Explain to the producer, engineer or owner that you are a professional singer who is able to fill in whenever needed. If you don't live near recording studios, you may want to seriously consider moving to one of the major music cities, such as Nashville or Memphis, or at least close to a major city likely to have at least a couple of studios. To be a studio singer and get work, you will need to be near recording studios.
Always be prepared for a live audition. If you get called by a recording studio for possible inclusion on a list of substitute vocalists or backing singers, someone from the studio will want to hear you sing.