Actually really easy.
Download any recording program ( I suggest using Audacity its free and it works, click link for free download ).
You need an Aux Cable.
Take the Aux cable and plug into your walkman or microcassette recorders headphone jack. Stick the other end of the Aux cable into the Mic jack on your computer.
Open Audacity and hit Record.
Then Press play on the walkman.
The Aux cable acts like a microphone but instead its playing the music right on to the recording program
record the entire tape/demo all the way through
Open a new file in audacity keeping the entire recorded tape/demo open in another window
just cut track by track, saving as you put each track in the new file
Its best to mixdown right to WAV file instead of saving each track as its own file
or buy this for 150$
Tape to MP3
brett kaplan told me about this in 1999 it just hit me a week ago