Actually.... it's real simple... it's the "recognizing" that's hard!
It used to be that mastering involved the art/science of capturing/shaping a mix to obtain the best possible sound and best possible shaping to have the disc cut on a lathe...
Now of course, we don't have to worry about it as much (the lathe-cutting, that is) and mastering has become the art/science of preparing and polishing a mix for release.
In actuality, all you really need to do this is an excellent 2-channel signal chain and a pair of excellent ears. You listen to a track and decide objectively how it sounds -- too bright? too dull? too boomy? no bottom end?? no top end?? some parts of the song are harsh? are there transient peaks that are causing undue distortion? etc...
These are the listening qualities you need to be able to analyze and quantify. Once you've done this, you need the tools to make the fixes...
Again, this can be the hard part - the cardinal rule in mastering is to make it "better than the original" - anything you do that doesn't meet this is counterproductive... with that objective in mind, obviously, you can't introduce ANY element into the signal chain that doesn't meet this requirement. So throwing
an Alesis MEQ230 into the chain is NOT a good idea!
Next is "how to make it bigger and better" -- this takes some experience more than anything else... need louder level? sure you could slap a compressor on it - go too far and you've lost the dynamics - use a bad compressor and you've colored the signal, breaking the cardinal rule.
So you need to experiment - having poor gear to work with just makes life harder, since, even if you have the unit bypassed - just having it in the signal chain will break the #1 rule.
Expanding this concept - you also need to worry about mundane things like fade-ins, fade-outs, cleaning up strange noises... all the little tweaks that you'd expect from a professional product. And if you're working on a series of tracks (such as on a cd) you need to deal with levels between tracks, tonal/timbral consistency between tracks, and general overall "even-ness".
What a pro mastering engineer can add to this is the objectivity of having done many such cuts, and therefore be able to statistically analyze your tracks in the context of everyone else's - making sure that yours "fit in".... this is a very important point in the analysis process and what is impossible for the DIYer to do themselves. (Which is why I'm always going on about professional mastering...)
If someone has done ALL the tracking, ALL the mixing, ALL the production -- they are the very last person that should be trying to be objective about their work... it's simply impossible to do. So their mastering analysis of what the track(s) need would be wrong from the git-go..........
Anyways - hope this helped...
(YMMV... batteries not included... not responsible for any lost or stolen articles.... yadda-yadda....)
Bruce