Get the mix as near perfect as possible first, mastering is gentle final touches, not fixing major problems in balance between instruments/voice or eq. So mix into a gentle compressor on the master bus with a ratio of around 1-2 or maybe 1-4 first. Then export the stereo mixdown. Do the same with any other tracks you need to give a similar sound to (e.g. because they'll all end up on the same CD).
Wait a few days at least so the next stage can be approached with a head clear of what you were thinking when you did the mixing.
Set up a new project and import all tracks that need mastering, so each occupies a separate section of the time line. Listen to the lot. If anything strikes you as wrong go back to the mix and sort it out. Also load a reference track either one of your own that you are happy with or a commercial mix that has the kind of sound you're aiming for. Route it directly to a hardware output so it doesn't go through the master bus and use it as a comparator with your own work.
Insert onto the master bus a multiband compressor, eq, maybe a reverb with a very short tail set to almost entirely dry (can be a useful "glue"). Then a frequency analyser (Voxengo Span is good and free) and a gonomiometer to check phasing. Finally a transparent limiter to use for the final volume setting, limiter set to not allow anything louder than -0.3dB.
Set all track and bus meters to show RMS as well as peak values.
Proceed from there. There are as many approaches to mastering as there are people who do it. Reading up on Bob Katz's K system is a good idea, as is watching some of the more professional videos on Youtube (the Point Blank FFL! ones are useful). Finally, it helps if you have a decibel meter so you can set your monitors to a consistent level somewhere between 78 and 85dBA (our hearing system is easily fooled and tends to think a slight volume increase = better sounding even when it isn't).
Take lots of breaks to avoid hearing shifts and tired ears.