Familiarize yourself with your mixer's equalizer. Most equalizers consist of multiple "bands," which cover low, middle and high frequencies. Some even offer bands between the three main frequency types. Refer to your mixer's manual to see how your specific equalizer is configured.
Set your equalizer to match the range needed for the specific type of vocals you are fitting into your mix. Bass vocals should be set to 75Hz at the low point in the frequency range and 300kHz at the higher range, whereas baritone vocals should be set between 100hz and 400kHz. Tenor vocals should be set between 135Hz and 500Hz, alto at 180Hz to 700Hz, and soprano at 250Hz to 1100Hz.
Tweak the equalizer's settings to remove undesired frequencies from your vocals. Cut the EQ band one or two decibels around the 1kHz mark. Cut all frequencies below 80Hz to eliminate muddiness. Add presence to the vocals by boosting the band within the 4 kHz-6 kHz range.
Adjust the equalizer's settings to taste. Bypass your EQ to get an idea of how the vocals sound before effecting them, and then re-enable the EQ to continue modifying the sound. It is essential for recording engineers to utilize their own ears for determining which frequencies work best for a given vocal track, after the basic frequency ranges have been determined by the EQ.