Import the audio file into a recording program such as Pro Tools, Apple Logic or GarageBand. Upon importing the file as a new track, you will see a waveform that represents the audio of this file.
Create a master buss. This is usually done by selecting "Create new track" and creating a stereo master buss.
Insert a compressor plug-in onto the master buss. A buss compressor such as the Waves SSL or API 2500 works well for this type of job, but any compressor will do.
Select a mastering preset from the plug-in's dropdown window, or adjust the threshold and compression ratio to get the desired amount of gain reduction. Aim for about 2 dBs of gain reduction with a compression ratio of 2:1 to 4:1 in order to get more transparent-sounding results.
Adjust the attack and release controls of the compressor to taste. In most cases, the attack should be set somewhere between 10 milliseconds and 40 milliseconds, and the release should be set somewhere between 300 milliseconds and 1.2 seconds for more transparent-sounding results.
Insert a limiter plug-in after the compressor plug-in on the master buss. The Waves L1 limiter or Digidesign's Maxim limiter work well for this task.
Adjust the threshold of the limiter to get the desired amount of gain reduction. Aim for a gain reduction of 2 to 5 dBs for more transparent results.
Adjust the make-up or output gain of the limiter to make up for the volume lost in gain reduction. If using the Waves L1 limiter or Maxim limiter, the make-up gain will automatically be adjusted to maximize the output of audio track.