Plug a guitar cord into the PA mixer input jack labeled "line in." Plug the other end of the cord into your guitar. Plug another guitar cord into the PA mixer's input jack labeled "line out." Plug the other end of the cord into your guitar amp as you would normally.
Turn on the mixer and the guitar amp. Set your guitar amp's controls as you would normally. Place a recording mike 6 inches away from the guitar amp pointed straight at the amp.
Play the guitar. On the mixer's control panel there will be lighted meters that bounce up and down when your guitar volume peaks. If any one of the meters bounces up into the red, turn the volume control down on the mixer until the meters never bounce into the red when you play. This equalizes clipping.
Play the guitar. If the treble is too sharp use the mixer's sliding equalizer buttons to bring it down. The PA equalizer is 12 small knobs that slide up and down. They will be marked "EQ" and will be located on the face of the mixer separate from the individual mike controls. From right to left, the far right buttons equalize the treble. Slide the button on the far right down one notch at a time until you get the treble equalized where you like it.
Play the guitar. If the guitar is booming with too much bass, slide the far left equalizer button down one notch at a time until the bass notes are equalized.
Play the guitar. If the guitar sounds weak or whimpy slide the middle two equalizer buttons up one notch at a time until the guitar sounds full and rich to equalize midrange tone.