Set the vocal monitors directly in front of the singers. The back of the microphone should point directly toward the stage monitor. If the singers use hand-held microphones, ask them not to hold the microphone down beside their body. This usually results in the front of the microphone pointing toward the monitor.
Set the volume for each microphone at a minimal level. Each microphone is plugged into a separate channel. Work on one microphone at a time. Mute all the microphones except the one that you are working on. Gradually increase the volume level. It is important for singers to clearly hear themselves, but try to keep the onstage volume level as low as possible. Feedback is a direct correlation to high volume. The higher a vocal monitor is turned up, the more potential for feedback.
Set the equalization -- EQ -- levels for each microphone on the mixing board. The EQ controls include treble, bass, and mid-range frequencies. The acoustics of each venue are different and the tonal properties of each singer's voice vary as well. The EQ sliders on the mixing board allow you to adjust the sound for each voice. Depending on the venue and the voice, this involves adding more highs or more lows to thicken the sound. The treble, mid-range, and bass frequencies also are potential feedback problems. Feedback usually begins slowly and builds up into a screeching sound. Determine if the lows or highs are causing the problem and back down the appropriate slider until the feedback dissipates.