Insert the cassette to be recorded into a tape deck. Attach the stereo RCA cable's male red and white plugs into the corresponding red and white output sockets in the back of the cassette deck.
Insert the mini-jack into the headphone socket if using a laptop, or the "Line-in" socket on the back of a desktop computer.
Open your music-recording software. This can be anything from Garage Band to Reason, but obviously the better the program, the better the sound quality. Access the music-recording function, and set the program up to record a new track.
Press "Play" on the tape deck and adjust the volume levels so that the input level gauge on your computer software reaches near the top, but not into the red part of the gauge bar since this means that the recording will distort.
Rewind your tape, hit the "Record" button on your computer software---usually denoted as a red circle---and press "Play" on your cassette deck.