Turn off the stereo, and ensure that the amplifier is off as well. Hook up the guitar amplifier to the stereo by inserting the cable in the line-out jack on the back, or side, of the amplifier, and then plugging the two red and white cable ends into the corresponding red and white female plug ports on the back of the stereo. The plug ports on your stereo should be labeled "Aux," for auxiliary, or "Input device."
Plug the guitar into the amplifier using the amplifier cable. All models of amplifier and guitar will vary slightly.
Turn the volume for the stereo to the minimum setting, and then turn on the stereo. Ensure that you place the stereo on the auxiliary or input device setting.
Turn on the amplifier, and then slowly turn up the volume on the stereo while strumming the strings until you hear sound. Adjust the volume controls on both the amp and the stereo until you've achieved a quality sound with the least amount of static.