Connect all the cables from the instruments to the monitors or PA system. Most mixers have inputs for XLR cables, ¼-inch instrument cables and RCA cables. Each instrument will be assigned to the channel of the input into which they are plugged.
Adjust the sliders with the band playing to get an even and desirable mix of instrument sounds. Watch your meters to make sure you don’t have any instrument’s volume, or your master volume, in the red area of the meter.
Twist the equalization knobs to adjust frequencies of each instrument. Most mixers will have at least three knobs -- one for bass, midrange and treble. Particular areas to listen for when adjusting equalization are the bass guitar and bass drum, as they exist in the same frequency range and can get muddy when played together without proper equalization. Guitars and vocals that occupy the same frequency range might also need to be adjusted.
Twist the pan knobs to adjust the instruments in the stereo spectrum. For instance, if a band has two guitar players, the sound guy might pan one guitar all the way to the left and the other all the way to the right. Some sounds guys also pan the drums across the stereo spectrum so that when a drum does a fill across the toms, the sound moves from one side of the speakers to the other.
Add effects to channels as needed. Some mixers come with built-in effects such as reverb or compression. To further customize the sound on each individual channel, you can add these effects, a different level on each channel, to achieve the most desirable mix possible.