Very simple. Take the headphone output of the MD and plug it into the Line In socket on your soundcard.

Make sure you start with the MD volume very low, and the Line In level on your PC quite high, then gradually bring the MD volume up to an acceptable level without distortion.

You may need to go into your PC volume controls to set things up - right click on the loudspeaker icon in the tool tray, and select 'Open Volume Controls' - you may find the line input is muted. You may also need to go into 'Options' and select 'Properties' and 'Adjust Volume for'... recording.

Once all this is sorted, you can use 'Sound Recorder', which is part of Windows, or a utility supplied with your soundcard, to record the tracks.

If you are really serious, you need to find some decent software such as Sound Forge or Cool Edit...