Set the master volume dial to zero, then turn on the mixer. This is the safest way to turn on a mixer. If there is a loose or damaged cable, this process prevents any buzzing sounds from being sent to the amplifier.
Balance the level of each singer individually. It doesn't matter which singer you start with. Each singer's microphone connects to its own mixer channel. Slide the "Gain" dial for the relevant mixer channel up. Gain refers to the output of a specific signal relative to the master volume. Keep moving it up until the red warning light flashes on the volume unit's meter. This is the maximum volume level before distortion.
Reduce the gain level slightly until the flashing recedes. The space between the fader's current position and the maximum level is called "headroom." Because the master volume is at zero, you won't hear anything. Just use the volume unit meter as a guide.
Fade up the "Master Volume" dial just enough so you can hear the amplified voice over the actual voice. This way you are dealing only with the microphone output. You set the master level later.
Tweak the collective levels. Have the singers perform in ensemble. Slide the fader dials up and down to compensate for natural projection levels. For example, some singers may have naturally louder voices, so fade their "Gain" dial slightly to temper the volume.
Increase the lead vocal level so it is slightly louder than the rest. Use the headroom space to achieve a slightly higher level.
Adjust the master volume in relation to other sound sources, such as a backing band or backing music. The method for tweaking the master volume varies according to the other sounds on stage. If there is a band, have them play and move the master volume slide up until all of the vocals are comfortably audible over the sound. If the backing track is coming through the mixer, too, adjust the "Gain" dial for the backing track level so it's slightly below the lowest individual vocal channel level.