In the main, if you can make a mix 1) balanced 2) clear and 3) loud enough, you'll have done 90% of the mixing:
Mix balance is achieved by shaping the mix like an equal loudness curve
Mix clarity is achieved by making separate audio dissimilar
Mix loudness is achieved by reducing dynamic range
In Practical Terms
In the main:
To make a mix balanced, pick a balanced reference track from your given genre or an equal loudness profile as a reference, and mix while targetting the reference's EQ shape.
Shape your mix to resemble the shape of a reference track or equal loudness profile
To make a mix clear, separate instruments by having them occupy different frequency ranges, or separate overlapping instruments by ducking levels (sidechaining).
Instruments sitting in different frequency zones
To make a mix louder, reduce the level of any elements or transients that are spikes in the mix
Illustration of a mix where the average track level (green) is quiet relative to 4 louder elements / spikes (red)
