The goal of a mix is to sound balanced and together as one unit. That takes a lot of practice and time.
The first key factors I look at are:
What instruments are there, how do they sound together just sound wise?
Are there vocals? How were they recorded? Do they need Pitch correction and Volume automations?
Are there drums? Do they sound as you want? Do they need to be edited in timing? In volume at places? Are some hits very soft and ones very loud?
How well does the Bas sound, does it work with the kick?
Usually when I start to mix a song my first main focus are the Bas, the Kick drum and the Snare drum. Once they’re at the place I will mix in the rest,I specialize a bit on vocal editing so if that has to be done, timing, pitch and volume wise ill do that. I use Melodyne for the pitch correction. Then I manually draw volume curves for the amplitudes and if needed, put alow working compressor on, ratio usually around 1.8:1 and the threshold and the rest where needed, I’ll also do a lookahead on the compressor to have it work instantly without any attack-time.
What effects does the producer or artist want?
I usually start with putting reverb on everything except the Bas and the Kick drum. On the reverb I often use 1-2s delay, 10-14 ms predelay, a high cut and a Low Pass filter. I also EQ out sub 150 Hz frequencies because they just muddy up the sound.
Then I will use effects as the song requires or the producer asks for and mix and bounce the song or stems for you.
Ridley Music Production 