Slide the equalizer gain slider to zero. The equalizer interface on the mixing board is governed by a master volume. The master volume controls the level of the outbound signal. Before you begin equalizing the vocal, minimize the gain. This gives you space to gradually increase it as you go.
Set all frequency band sliders to the center. When a slider is in the center of its range, it is neither boosting nor decreasing the volume of that frequency.
Fade the gain slider dial up while the vocalist performs. The equalizer adds a slight boost to the overall signal. As you slide the equalizer master gain up, move the mixing board master volume dial to compensate for the increase.
Tweak each frequency slider in turn, starting with the lowest frequency band. The lowest band, which is typically between 16kHz and 20kHz, is always furthest left. These are the bass frequencies. If you aren't experienced enough to know which frequencies need adjusting by listening, monitor the effect of each slider adjustment. If the singer's voice is too explosive, or "popping" when he makes a "P" or "B" sound, reduce the level of the 80kHz frequency.
Monitor the mix from a different location. If the board is at the back of the venue or at the front of the studio, you'll get a different perspective by moving to the opposite end of the room. Remain central to the two speakers.
Compare the equalized mix with the dry mix. Dip the equalizer gain to hear the vocal without equalization. Then boost the gain again to hear the contrast between natural vocals and equalized vocals. The equalized vocals should be crisper, with more clarity.
Tweak the equalizer mix. Adjustments made to high frequencies can leave space in the mix that highlights flaws in other frequencies. For example, decreasing a high frequency can cause a previously inaudible frequency to be heard. Make small adjustments to each slider once you've completed your first overall adjustment.