Before getting into more detail, there are some ground rules to be covered. All of these are important guidelines for any mastering engineer. I'll spread them over a few posts, but we'll start with rules for before you start mastering.
1: Always use a fixed listening environment
Which means level, room, monitors - the whole thing. Mastering is the art of turning a collection of pieces, songs, tracks, into an album. (Or single, or EP...) We're trying to judge impact, pace, atmosphere, dynamics and timing ( amongst others ) - in fact, as a professional mastering engineer I'm almost trying to second-guess the artist. To make a fair and accurate judgement compared with all those other discs, we have to be 100% confident we know what we're listening to, which means eliminating variables in the listening environment. The same goes for any room used for mastering - it needs to be as good as possible.
I'm going to cover room acoustics and treatment in a later post, but the first and most significant thing to control is the level - ie. how loud you listen to the music. I have two levels I master at - one is 12 dB below the other, and I switch between them throughout a session. The exact level you use isn't critical, but it needs to be consistent. (For the technically minded, use an SPL meter and generate pink noise at -18 dBFS. In my room this corresponds to a C-weighted SPL of 79 dB SPL.)
Then listen long and hard to some good CDs. Figure out which ones you consider to sound perfect, which ones sound too loud, which too soft, which too bassy, which too bright. You might decide to use a slightly higher or lower level than I've suggested to suit your comfort or taste, but there are very good reasons for sticking close to this reference level.
You'll probably find that a large number of recently mastered "classic" albums sound absolutely fantastic at this level, and also that many of the newest sound uncomfortably loud - check out my first DIY post for more about loudness.
Once you've settled on a level - stick with it. If you work at it for long enough you'll develop an instinct for the perfect level for any piece of music, and more importantly you'll avoid being tricked by the Fletcher-Munsen effect; in a nutshell, quiet sounds appear duller and thinner - ie. if you master with your level too low the chances are you'll add too much bass or top, and vica versa.
2: Use full-range, FLAT monitors
A CD is capable of reproducing a frequency-range of 20 Hz to 20 kHz. Your monitors need to come close to this - anything they can't reproduce, you can't hear, and may misjudge. In a pro mastering room this generally means the monitors will be big, and expensive. (In most mastering-houses they may easily cost more than a family car !) So try to choose neutral, accurate speakers, with a flat frequency response - perhaps bland-sounding to some ears, because that way you know it's the music that sounds great, not the monitoring. B&W make some superb hi-fi speakers which could be suitable for DIY mastering.
Ideally you should master on different monitors than you mix on, and even in a different room. This is partly to get a fresh perspective on what you're listening to, but also it means that if there are problems or limitations with the monitors you mixed on, you are more likely to spot this. If you're going the DIY route obviously this won't be easy, but the next best thing is to listen to your mastered audio in as many different places as possible. If it consistently sounds boomy, it probably has too much bass. But problems that you only hear on one system are more likely to be to do with the setup of that system.
One good tip is to setup your home stereo speakers in the mastering room, assuming they're decent quality - since you listen to most of your music on these, you may be much more sensitive to mastering judgements listening to them. Even listening on a favourite pair of headphones can be useful. But remember the golden rule - pick two mastering levels (three at most) and only use these when mastering.
3. Use a loudness meter
As I said in my first DIY post, perhaps the most important thing a mastering engineer does (apart from put the right tracks on the CD, in the right order !) is to choose a consistent, musical level for each track. The most important tool for deciding this is your ears, but meters giving you an objective measurement of the level are invaluable. Most people will probably find it easier to use the digital metering offered by a piece of software, but I still have a soft spot for the analogue needle, or VU meter.
Whichever you decide on, the most important think to know is why bother to use a loudness meter ? Because the absolute and relative loudness' of tracks on an album have an immense effect on their perceived sound and a peak meter is virtually useless for judging loudness. The classic example is the human voice - a very quiet voice can have an extremely high peak level. It "looks" loud on a digital meter, but it sounds quiet. A VU or digital loudness meter looks much more as things sound in terms of level. If you learn how the meter relates to loudness it'll help you make good judgements in "mastering".
More guidelines in a later post...