How do you separate the instrumental part of a track from the vocals on a song?

There are programs you can download online

You would need the master track to actually make it sound normal.

they have to be recorded on different mics and saved on different tracks.

record it separately. the sound you hear is a combined sound, the only separation you can really do after the signals have been combined is highs/lows and frequency related signals. But this doesn't gaurentee you can separate just what you want