Mixing down a song entails making each element sound great on its own and in ensemble with the other tracks, each in its appropriate place left/right/center/front/back as heard by the listener, with special equalization or other processing enhancements that clarify the meaning of the lyrics and perhaps the drama of the instrumentation, with original and creative effects applied as necessary, with the view toward maximizing the emotional impact of the song, setting new trends in popular music and winning Grammys, then preserving it in the correct final format in 2 stereo tracks without pissing off the talent or the writer or the producer or yourself. In doing this you are, of course, limited to what pitiful little collection of hardware and software you have managed to accumulate.
That's all there is to it, although there are several here who might want to add some other factors.