Mastering is perhaps the least-understood part of the music production process. The way people talk about mastering would lead you to believe that it’s somewhere between cold fusion and alchemy on the spectrum of “stuff that I don’t really understand but seems like it’s really important.” But as the chart above shows, it’s really not that complicated: at its core, mastering is just controlling frequencies (using your old friend the EQ) and dynamics (using a compressor and limiter).
As producer Jesse Cannon (Man Overboard, Transit, The Menzingers) shows in his class “DIY Mastering,” a key part of the mastering toolbox is the master bus compressor aka buss compressor or glue compressor. Added to your master bus, it does a few important things: “glues” the elements of your mix together, makes everything a little punchier and smoother, and tames any out-of-control peaks (such as random snare hits, bass drops, and other highly transient elements).
As far as plugins go, there are plenty of options ranging from Flux Solera at the high end to Massey CT4 at the low end (free). Our go-to bus compressor is Waves SSL, which is right in the middle in terms of price. While you can always go harder, start with a fairly gentle ratio of 2:1 or 4:1 and adjust the threshold until you get about 2dB of gain reduction. Attack and release will depend on the song, so just adjust those to taste. Slower attack times will let more of the transient through and allow for more dynamics, but your music may have some wild transients that need to be tamed with faster attack. And if you’re wondering what the “analog” button does, it just adds faux tape hiss. For more modern genres like metal or bass music, you’ll probably want that off.
Check out this video for more specifics: