That’s actually very easy to do and you don’t need any special equipment. Simply use a male-male 3.5mm cable and connect one end from the stereo output of the cassette player and the other end into the microphone jack of any computer you own. Play the cassette - you can test the audio quality by running arecord -f cd - | aplay - - you will have to tune the volume output of the cassette player and the input sensitivity of the microphone.

From there, if you’re paranoid, you could use arecord to save the output to a .wav file and encode it once the recording is done, but I had no problem just using oggenc directly on the piped audio. The final command looked like this: arecord -f cd - | oggenc -q 5 -o file.ogg - (change to -q 10 if you want lossless encoding).

I’m not sure if this is the best quality per se, but I would definitely recommend it over using specialized equipment like cassette-mp3 converters. The problem with those devices is that if they use underpowered hardware, you might experience buffering issues where the encoding hardware can’t keep up with the audio stream or something like that. But doing it on a computer ensures that you will have all the processing power you need to make sure that this doesn’t happen.

Good luck! I found it very easy to do - it took 5-10 minutes of setup.