Choose an appropriate location for the monitor mixer. Typically, this is on or to one side of the stage area, behind the main speaker banks.
Connect the stage monitor speakers to the appropriate outputs on the monitor mixer. Depending on the situation, the number of stage monitor speakers could range from a single speaker to a dozen, or more.
Connect the instrument and microphone feeds to the appropriate inputs on the monitor mixer. These may come directly from the stage or as a output from the main or front-of-house mixing board.
Set the monitor mixer to default settings, also know as "zeroing out the board". This is accomplished by reducing all volume and effect level controllers to zero or off, and setting all equalizer controllers to neutral or a "flat" setting. This ensures that the only changes to the mixer settings are the ones you initiate.
Adjust the mix of instruments and volume level of each to suit the performance, using each monitor. As opposed to the music heard by the audience, each performer may only want to hear his or her own performance mixed with a limited number of other performers. For example, a bass guitar player may only want to hear his guitar, the snare drum, and the lead singer in his monitor mix. A drummer may want to hear the guitarist, bassist, and lead singer in his monitor mix. This is the part that gets tricky and may require some extensive adjustments before the desired levels are achieved. Work with each performer to make sure they are happy with their monitor mix.