I recently took a study break after high school graduation to pursue my lifelong dream of becoming a music icon after bumping into another superstar friend. Now I do not want to consult him on my music, but at the same time, I want help regarding music and how to mix and master it.
With multi-track recording most of the work is done during mixing… Balancing the levels of the various tracks, applying effects, editing-out errors, applying Auto-Tune, etc.
Mastering is just adding “final tweaks” and creating various formats for distribution after you have a stereo (or surround sound) mix. There is usually some level adjustment, additional EQ, dynamic compression and limiting, and possibly reverb.
Usually it’s done by a different engineer with “fresh different ears” in a different studio with different speakers, etc.
But if you do it yourself, presumably your mix already sounds as good as you can make it (and you may not want a mastering engineer messing with it). But it’s still usually worthwhile to at-least adjust the levels after you have a “finished stereo mix” because it’s hard to exactly predict & adjust the levels of the mix before it’s done.
Mastering is also where the dynamics are destroyed with excessive compression and limiting to “win” The Loudness War. (Mastering has developed a somewhat-bad reputation over the last several years.)
I don’t have anything handy about mixing but here are a few mastering links I’ve collected:
Izotope - Ozone Mastering Guide (2022 -This is written for Ozone but most of it can apply no matter what tools you are using.)
REAPER Forum - Discussion of a particular mastering job (2008 The mastering discussion starts at post #8)
Mastered For iTunes (2012)
Audio Mastering (Bob Katz)