Connect your stereo amplifier output to your computer using an RCA-to-1/8-inch headphone jack cable. Find the cassette deck on your stereo and locate the headphone output plug. Insert one end of the cable here and the other end into the sound card slot on your computer.
Select a bitrate in your MP3 encoder, such as Audacity, RealOne Rhapsody or MusicMatch. For most users, 128 Kbps will work just fine because it provides CD-quality sound. If you want higher quality, select 160 or 192 Kbps. Higher quality takes up more space on your computer drive, but if it's important to you, do it.
Start recording. Click "Option" from your MP3 encoder menu and set the recording source to "Line In." Adjust the recording volume by opening "Control Panel," then "Sounds and Audio Devices." Use your mouse to increase or decrease the recording volume level.
Locate your new MP3 file on your computer. From your MP3 encoder, click "Options," "Settings," "Recorder" and look for the Tracks Directory. The directory will show you where the file is stored on your computer so that you can locate it, open it and listen to it. You can also move the file to another, more convenient location, if desired.