Solo all the vocal tracks in the project (or mute all the non-vocal tracks) to isolate the vocal performance.
Create a new track to set up an auxiliary bus containing mastering components. Name the track "Bus 1." Add a compressor and equalizer to Bus 1.
Send all of the vocal tracks to "Bus 1" to place the compressor and equalizer in the signal path. The vocals are now affected by the mastering components.
Play the audio and watch the compressor meter. Adjust the settings. Threshold determines the decibel level at which the compression begins to work. Ratio determines the amount of compression applied once the audio has crossed the threshold. Attack determines the time it takes for the compression to begin working once it has crossed the threshold, and release determines the time it takes for the compression to stop working once it has left the threshold. Adjust the threshold, ratio, attack and release until the vocals play back at a consistent volume.
Adjust the equalizer to your liking. Different vocalists require different EQ adjustments. In general, a high pass filter at 60Hz removes unused low frequencies, a subtle boost between 200Hz and 600Hz adds body, a small boost above 6KHz adds brightness and a cut between 1KHz and 2KHz smooths vocal performances.
Adjust the fader on the auxiliary bus to increase or decrease the amount of equalization and compression affecting the vocal tracks.
Export the soloed tracks as a single piece of audio to render the mastered vocals.