Connect one end of the audio cable to the line out or headphones jack on your cassette player.
Connect the other end to the line-in jack on your computer. You can usually find this jack on the rear of your computer tower, near the speaker connection. If your computer doesn't have a line-in jack, use the microphone jack (quality will be reduced).
Insert your cassette into the player. Open your volume mixer and select "line-in volume" or "microphone volume." Make sure the volume is turned on and adjust it as necessary. If you don't see either setting, go to "configuration" and enable one of the lines.
Begin playing your cassette. You should be able to hear it through your computer speakers.