Set the knob labeled "gain" for the first instrument. Aim for a quality of sound with which you are happy. Gain should not be used as simply a volume control, but rather to affect the quality of the tone that comes from the channel. Set the volume at an approximation of your live volume and then set the gain. Make sure the meter doesn't peak much higher than a decibel rating of zero.
Adjust the equalizer levels for the instrument for which you have set the gain. Depending on the type of board you are working with, you might have a 4-band, 5-band or even larger-spectrum equalizers with which to work. Adjust the equalizers until you are happy with the tone.
Repeat this process for every mic and instrument that you have on stage.
Ask the band to run through a song. As they do, adjust the primary output faders to mix the instruments together. The key is to make sure that all instruments can be heard clearly without obstructing others. Often, the difficulty is to find the correct volume for the lead vocalist, so that he can be heard clearly above louder instruments such as the drums and guitar.
Adjust the monitor output levels. Ask each artist during sound check to indicate whether or not they need more volume from their monitors, based on the levels you have set for the live sound. Adjust the monitor output for each channel until the artists are happy with what they hear.