All correct except you need to plug into the microphone jack on your computer. Since headphones and microphones use the same size of plug some cables and computers are color coded with pink for microphone and green for headphone/speakers. And of course you need a male-male 3.5mm audio cable to do this.
You need a program for that ... I think "Audactiy" can do it. Anyway, you need to connect from the headphone jack of your cassette player to your computer's headphone jack (the pink slot - I think)
Thanks for the correction :)