Turn each volume fader on the mixing console down to zero. This will allow you to avoid damaging your speakers as instruments are plugged into the mixing board.
Plug each instrument into an open input channel on the sound mixer using either a 1/4-inch or XLR cable.
Get levels for each instrument. Have each musician play his instrument individually. As the player performs, turn up the gain knob on the corresponding input channel until the fader is close to, but does not exceed, the clipping threshold. If the signal is turned up too high the audio will overload. This will be indicated by the level meter turning red or a clip indicator light turning red.
Turn up the volume fader for each instrument channel and get a general mix of all of the musical elements.
Add EQ by adjusting the lo, mid and high EQ knobs on the sound mixer. Once you have a general mix going, add EQ (equalization) as necessary in order to bring out the unique characteristics of each instrument. For instance, you may want to add EQ to a bass guitar to give it more bottom or you may add more midrange to a guitar in order to make it cut through the mix better.
Add effects to instruments if needed. For example, you may want to add reverb to the vocals or delay to a guitar. Be tasteful. Add effects only as necessary and appropriate for the music style and artist you are mixing. Adding too many effects can quickly make a mix sound cluttered and amateurish.
Set the final output volume. Turn up the main output fader on the sound mixer until you have reached the appropriate volume for the venue.