Determine the needs of your band based on your musical style(s).
Place an ad in the music section of your local or regional newspaper stating dates and times for a backup singer audition. State clearly what musical style your band is playing, how many singers you want to add, and if you are looking for male and/or female singers.
Find a place that is appropriate for auditioning; for instance, the room where your combo usually meets for rehearsals.
Have your band and your lead singer present at the time of the audition.
Interview each applicant separately about his or her background and experience in the style you are looking for.
Select one song from your band repertoire and have each singer have a try at it.
Have the singers perform separately to test their vocal qualities.
After individual auditions, have the singers (usually in groups of two to four) perform together to see how they adapt to ensemble work.
Listen to whether they can sing in key.
Listen to each singer's ability to harmonize; that is, sing the same tune in different pitches.
Listen to the tonal quality of each singer.
Find out their talent levels for tuning and intonation.
Pay attention to whether his or her vocal range matches that of the other singers.
Have the backup singers join your lead singer. It's the only way to find out if their voices will be a good match.
Make sure they know basic sight-reading if that's required to join your group.