For the purposes of this Tips series, I define “Mixing” to begin once all of the tracks (vocals, guitars, keyboards, drums, bass, etc.) have been recorded, or “tracked” as most pro engineers refer to it. So that’s my starting point.
The first thing to do in my opinion is get the relative volume levels right between the various tracks. Engineers typically just call these volumes “levels”. There is no hard and fast rule for the order/sequence of the parts of the mix for which you set levels. For lots of common types and genres of songs, a good place to start is the relative levels of the rhythm section (bass and drums), then start on making those level decisions for the prominent instruments (e.g. guitars if your song is guitar-heavy).
Getting the levels right should come before anything else, including “panning” left and right, although as you get close there’s no harm in beginning to fold in those decisions too. If the song is not an instrumental, be sure your vocal tracks are the center of attention, because the vocal is usually the centerpiece of the song.
And as the tip directly above this one says, using a reference track is critical, especially at this stage right here. Using a reference track means being able to switch instantly between your mix and a commercial song of a similar style or genre to what you are doing. (Switching back and forth instantly is referred to as “A/B” listening, because one wants to switch between thing A – your mix – and thing B – the commercial song – at the touch of a button or click of a mouse.) Listen critically: How loud is the bass compared to the drums in your mix vs. the commercial song? Do your vocal tracks stand out the way they do in the reference song? Do the guitars cut through to take center stage where they’re supposed to, or are they hard to pick out?
The tip directly above also gets a critical point, that listening attentively is absolutely crucial. It takes practice. Don’t be frustrated if it doesn’t click for you right away.
So get those great-quality tracks during the Recording phase, then when you first start mixing, get those levels set properly, and use an appropriate reference track to make those decisions. The rest of your work on the mix will benefit greatly from these first steps.