Make sure your cassette deck and computer are plugged in and turned on. Rewind your cassette to the beginning.
Plug the red and white RCA ends of the converter cable into their color-matched "Line Out" outputs on the tape deck. The white plug represents the left channel and the red plug represents the right, and it's imperative that you plug them into the correct outputs in order to avoid a reversed stereo mix on your MP3s. Plug the 1/8-inch side of the cable into the "Line In" jack on your computer.
Press "Play" on the tape deck to get a sample level. You should hear the music coming through your computer speakers or headphones.
Find your computer's "Line In" recording level via the "Sound" options on the control panel. (This location varies by computer and operating system but should be easy to locate.) Use the slider to adjust your "Line In" level so that the meters are reaching the yellow area but not the red area. If the levels reach the red area, you might hear popping or distortion on your MP3s.
Stop the tape and rewind it to the beginning.
Open your music-recording software and start a new file. Press "Record" on the software's interface.
Press "Play" on the tape deck.
Wait until the entire side of the cassette is complete, then stop the software recording.
Name and save the file, then repeat the process for the second side of the tape.