Assemble your equipment. Locate the "Line Out" jack on your cassette player (or turntable, if you're converting vinyl) and the "Line In" jack on your computer. Make sure you have cables that can connect one to the other; if the jacks are of differing size or type, use adapters or buy a cable that converts from one to the other. (Your local electronics retailer can usually help you.) Use the cable to connect your music player to your computer.
Open the audio-editing software and make sure it is configured to receive an external signal. Test the connection by playing the audio; if you can hear it through the computer speakers and/or it registers a visible signal on the software display, it is working. This step may take several attempts, so be patient. Once you have a connection, set input levels so they are strong but not distorted.
Record your audio, one track at a time. Press "Record" first on the software, then play the track. Press "Stop" when the track concludes. Save this file as a WAV file for now.
Use the editing software to eliminate any "hisses" and "pops" if you desire. Many types of software have a function that does this automatically. You can also remove silences at the beginning and end of the track. Don't save changes until you are satisfied with the results, or save a working copy.
Convert the file from a WAV file to an MP3 file using the editing software. Refer to your software's help section if necessary. If you simply plan to burn the file to CD, you can skip this step for now.
Repeat Steps 1 to 5 for each track you want to convert. If you are burning a CD of the audio, use your computer's CD-burning software to burn the WAV files to an audio CD.