The way I like to view a mix is to convert it from an audio concept to a visual one. When it comes down to it, creating a mix is not unlike painting a picture. We have three dimensions involved: frequency, pan, and depth.
Frequency, the vertical part of the graphic, is the section of our hearing the the instrument fills. The most important part of creating a mix is making sure the various instruments don’t overlap with each other in frequency. If we pull that off, the rest of the mix is quite easy.
The horizontal part is pan, which most of the time is experienced as left/right separation in a stereo mix. While it can sometimes cover up overlap in the frequency domain, pan is best used for creating body and fullness to a mix, making it sound more “live” in the studio.
The final dimension, depth, is the most nuanced; it’s a combination of volume and effects. For example, a loud (in the mix) vocal with too much reverb sounds farther away than a quieter snare with no reverb at all. It is represented by the size of the object. A mistake in any one of these three dimensions can destroy an otherwise good mix, but when all three work together the rest is a triumph of art and science.
So, awful text representation aside, here is how I create a mix, in visual form:
As you can see, I’m assuming a fairly standard “rock” group here, but the basic principles apply to any style. The lead instrument, in this case the vocal, is front and center, very present in the mix. When another instrument is the lead, it supplants the vocal’s place; a guitar solo, for example, would mean the lead guitar taking the spot of the vocal.