Acquire and install mastering software on your computer (see Resources).
Open the mastering program and load the track that you wish to master. Turn the track's volume to maximum to ensure that adequate headroom exists for mastering. In other words, enough leeway should be in the mix to allow for loud sounds to occur without sending the mixer's levels into the red zone.
Begin by adding compression to the track. Compressing a track will marry each individual piece together, resulting in a cohesive whole. Turn the "Attack" and "Release" knobs toward the high position.
Determine whether your track needs equalization. For example, you may need to add some low end if the original track sounds tinny. If your original track sounds muffled, add high end. The goal is to achieve a balanced sound.
Remove all frequencies below 30 Hz. This will allow the instruments in the track to interact harmoniously while negating any rumble.
Apply brick-wall limiting to the track by using a mastering limiter, which is an included plug-in with any mastering software. Adding limiting (as well as input gain) will enhance the volume in the mix, resulting in what many people refer to as "the mainstream effect," so named because it resembles the sound commonly associated with commercial music.
Trim the beginning and end of the track and export to the file format of your choice (most often MP3).