Lower the music on the track to hear the vocals properly before adding any compression. This way you will be able to hear the peaks and valleys in the overall volume of the performance. Compression will raise the lower volume passages as well as lower the louder sections to give your vocals a more even, powerful sound.
Set the threshold for the compression. Anything below the threshold will be unaffected and remain at the same volume, while the sound above it will be affected by the compression amount. Set your threshold lower for rock and metal, as you want the dynamics to be less obvious in louder songs. If you only wish to lower the peaks, set a higher threshold, as only the louder passages will be compressed.
Adjust the ratio of compression. This is the amount of compression used on the sound above the threshold. For a subtle curtailing of peaks, set the ratio low, such as 2:1. This is a very subtle setting, and most vocals are compressed at 5:1. This will allow the vocals to sound consistent and even throughout the entire song without "squashing" the sound. Anything above 10:1 will be very noticeable and destroy the dynamics of the vocal track.
Set the attack of the effect. This is how long it will take for the compressor to act on the sounds that are above the threshold. A lower setting will allow the compressor to kick on very quickly, but may make the effect more noticeable. By setting the attack too soon, you may get an unnatural sound to the passages being affected. By setting it too slow, you may get some louder initial phrases, which will make the compression too obvious when it kicks in. It is a balancing act, and the right amount of attack can only be determined by trial and error.
Set the release. The release is the time it will take for the compressed sound to return to its normal gain levels. You do not want the compressor to remain on too long after the initial attack, which can affect the sound below the threshold. There is a fine balance that must be established between attack and release times for a natural-sounding vocal track.
Adjust the gain levels. When sound is compressed, it loses overall volume. The more the compression, the more gain you must add to make the vocal track be heard in the mix. If you lose 6db by compression, for example, use the gain setting to increase your compressed sound by 6db to compensate.