Because cummerbunds are waist coverings, they should be worn at the natural waist.  This means that the trousers should be worn around the level of the navel, and half the cummerbund should cover the shirt, and the other half the trousers.  Wearing a cummerbund too high or too low can ruin the aesthetic effect.  Lastly, and possibly this goes without saying, a cummerbund should always be paired with a bow tie, preferably one in a matching fabric.