Connect the output of your audio cassette player to the audio input of your Macintosh computer. Most cassette players have a stereo RCA output, and all Macintosh computers have a mini stereo one-eighth-inch input. Special stereo RCA to one-eighth-inch cables are available from most stores that sell home audio equipment or musical instruments and supplies.
Open the Garageband program on your Macintosh computer and turn the monitor section of the first track to the "on" position. The monitor mode is located on the track details of each track on the left side of the screen.
Play the cassette recording and watch the volume input meter at the bottom of the screen in Garageband. Listen to the recording for a few minutes and make sure the volume does not go above the 0dB mark on the light meter. If the meter goes above 0dB, you will hear distortion, which is generally considered unpleasant. Reduce the input volume on the meter in Garageband until the loudest portion of the recording is just below 0dB. This may take a few minutes of monitoring.
Hit the "record" button in Garageband and begin playing the cassette source material. Let the source material play until it has finished or you want to begin a new track. At that point, hit the "stop" button in Garageband and stop the cassette deck. Stopping the devices in this order will prevent a noise from being heard when the cassette machine stops.
Save the recording and transfer the song to iTunes or any other recording program if you want to make a CD or for easier listening. You can transfer any song from Garageband by clicking the "Share" tab at the top of the Garageband window and selecting "Send song to iTunes."
Repeat the process in a new recording session if you want to record multiple tracks.