Find a computer with headphone jack.
Get a cassette recorder with a microphone-input plug. This will be labeled "Microphone", "MIC IN", "REC", or just a picture of a microphone. You can find cassette recorders at your local second-hand store for cheap, though they still sell these at Meijer, Target and similar outlets, starting around $15.
Find a "male to male stereo audio cable". It must be stereo, it must be long enough to reach from one device to the other, and the plug ends must fit the corresponding female plugs of your PC and cassette recorder. The most common audio plug size for PCs is 3.5mm (1/8-inch). See your local Radio Shack for the cable, as well as converters if you need to change the plug size. A six-foot cable will run about $5.
You've got the equipment. The rest of the work is easier than finding spam email in your Inbox: Plug one end of the cable into your computer's headphone jack, and the other into the cassette recorder's "Microphone" jack. Press record on the cassette player, then have your PC play your MP3s, podcast, or music. You'll be making a recording with no outside interference, and the recorded sound quality on tape should sound almost as good as the original.