Load the audio. Double-click on the file icon for the song you wish to master. This will open the file in the program in which it was created (for example, Pro Tools or Cubase). If you wish to use a separate program for mastering, such as Adobe Audition, double-click on the desktop icon for that program. Once it is open, click "Import" and select the audio file from the drop down list.
Load the multi-band compressor. Select "Tools" from the menu. Typically, the tools menu for mastering is located in the top left corner of the interface. The multi-band compressor lets you boost the gain of the different frequency bands. Gain is the strength of the signal. The mutli-band compressor splits the audio into separate signals by frequency.
Equalize the frequency range. Typically the bands are split into four: "sub," "bass," "middle" and "treble." Hit play and slide the virtual slider dial for each band up to boost it and down to reduce it. The multi-band compressor gives a visual display of which frequencies are strongest. Reduce the the strongest frequencies and increase the weakest ones so that all are approximately the same.
Limit the dynamics. Click "Tools" and select "Bus Compressor." A compressor attenuates high volumes and boosts low volumes. It does so according to the parameters you set. The bus compressor limits the dynamics of the total audio, rather than individual audio tracks. Slide the "Threshold" slider up to select the minimum gain level at which the compressor is triggered. Setting this low makes for a wider range of dynamics; setting it high creates a narrow dynamic spectrum. Once you've set a compression range, boost the "Gain" slider as far as it will go before distorting. Distortion is indicated visually by a red flashing light. Once the LED flashes, reduce the gain slightly until it stops.