Mastering is a pretty wishy-washy process with no "one size fits all" setting. Different practices will be required in different tracks. Just the way it is. Now, if you're asking about what tools are usually used in mastering, that is a much easier question with a more straightforward answer. In mastering, you usually use compressors/limiters or perhaps some kind of multiband compressor (just dynamics processing in general) to get make the audio levels a little more uniform in your sound. Beforehand, you can always touch up the track with EQ (preferably parametric EQs) to get the exact tones you want or to get rid of piercing, annoying high frequencies. The sound you are going for in your stereo mix will determine what needs to be applied in your mastering stage.