It is important to know which frequencies are responsible for which sounds. 120Hz to 600Hz is generally responsible for low-end sounds like bass vocal, bass guitar or cello. Snare drums, electric guitars and other full-frequency instruments produce both low- and high-frequency sounds that fall between 600Hz and 3kHz. Sounds between 3kHz and 7kHz are generally reserved for cymbals and vocal sibilance. Generally, anything above 10kHz is too high-frequency to be very loud in a mix. A good example would be the very highest-frequency sound a cymbal makes.
Connect your audio source (such as a CD or MP3 player) to the input jack on the equalizer with the appropriate cabling. Most devices can be connected to an equalizer with a stereo RCA cable, though some others, like iPods, require special 1/8-inch to stereo RCA cables. These cables can be purchased in any music store or store that sells home audio equipment.
Set the bands of your equalizer to the "center" or "off" position, where the equalizer is not altering the sound of the source material in any way. Most equalizers have what is called a "center detent position," where each band clicks in place.
Listen to the audio you are trying to equalize and note the changes you want to make. If you are unsure which frequencies must be altered to achieve these changes, make simple notes like "too much mid-range" or "not enough low-frequency sound."
Try to equalize your audio by cutting frequencies instead of increasing others. For example, you may be able to add more relative high-end to a recording by simply decreasing some of the low- to mid-range frequencies. Your ability to do this will depend greatly on the type of equalizer you are using; a 20-band equalizer will allow more fine-tuning than a seven-band equalizer.
Check to make sure any equalization changes have not increased distortion in the source material. While an equalizer with an output meter is helpful for this, you can generally tell by simply listening to the recording. Pay very careful attention if you have increased high-end frequencies, as these tend to increase distortion levels much more than low frequencies.