Connect the RCA (red and white) cables to the cassette player's RCA audio out jacks.
Connect the other side of the RCA cables to the corresponding ports on the RCA to 3.5mm adapter.
Connect the adapter to the computer's "Line-in" jack on the sound card. It is usually blue.
Right click on the speaker icon on the computer's task bar and select "Recording Devices."
Insert a cassette with audio on it and hit play on the deck.
Highlight the "Line In" option and click configure.
Follow the onscreen instructions to configure the line-in audio levels.
Open the Audacity program.
Click on "Edit" and "Preferences."
Change the "Device:" under the Recording section to "Line-in" and change the "Channels:" setting to "2 (Stereo)."
Rewind or fast forward the cassette tape to right before the track you want to record as a MP3 file.
Click the Record (red dot) icon in Audacity.
Press "Play" on the cassette player.
Press the "Stop" button (yellow square) in Audacity when the track is completed.
Press "Stop" on the cassette player.
Go to "File" and "Export as MP3" in Audacity.
Locate a folder to save the MP3 to in the file browser.
Name the MP3 and leave the .mp3 file extension at the end.
Click "Save."
Repeat from Step 11 for additional files.