Download and install Audacity Music Editor by clicking the link from the Resources section below.
Download and install the LAME Encoder from the Resources link below. (This is required to convert your cassette recordings into mp3s.)
Connect your 1/8-inch audio cable from your cassette deck's "Line Out" or "Headphone" jack into your computer's "Line In" or "Microphone" jack.
Open Audacity Music Editor. Select "Preferences" from the toolbar at the top of the window and make sure that your "Playback" device and your "Recording" device are set to your computer's soundcard. Push "Play" on your cassette deck to begin testing the audio levels. If the sound coming through your PC speakers is too loud or too quiet, you can adjust the volume by sliding the "Plus/Minus" bar (located beneath the "Play," "Pause" and "Record" buttons) left or right, in accordance with your desire to increase or decrease playback volume.
Delete anything you've recorded while testing audio levels and return the beginning of the time scale (the measurements located on the top of the sound waves). Press "Play" on your cassette deck followed by the red, circular "Record" button on Audacity.
Delete any unnecessary space at the end of the recorded track by highlighting the empty area and clicking "Delete" on your computer's keyboard.
Select the entire track by clicking "Edit" from the toolbar menu, followed by "Select All." Select "Effects" from the toolbar menu next, and then choose "Noise Removal." This will help eliminate any static and tape hiss that came through during the recording process.
Select "File" from the toolbar, and then select "Export As mp3." Choose the destination folder for your saved song files. You can use the "My Music" folder or you can create a new folder to keep better track of the files you've converted, as opposed to the files you've downloaded or ripped from CD.