The cummerbund (or cumberbund) is a traditional alternative to a vest or waistcoat, first adopted by the British military during the occupation of India. Today, it is largely a piece of formalwear used in tuxedos or similarly dressy mens attire. Many men will have never seen one until they rent a tuxedo and may be confused about how to correctly wear it. It may seem an odd little bit of fabric, but it is quite easy to handle.
Dress in trousers and shirt first. The accompanying cummerbund is generally the colour of the larger colour scheme of an outfit (i.e. black, for a tuxedo), though sometimes it may be the accent colour. Make sure that your cummerbund is suited to what you are wearing and not a mistaken accessory.
Attach the cummerbund around the waist so that the pleats face up. This has traditionally been so that the pleats can serve as an alternate pocket, though this usage has fallen out of fashion as the cummerbund has become more esoteric. The cummerbund should fit comfortably, not too tight, and the attaching hook or knot should be around the small of your back. Wearing the cummerbund pleats-down has become acceptable, though it is much more common to wear pleats-up, and if you are in a particularly formal occasion pleats-up is still the preferred method.
The cummerbund in the larger ensemble
Apply your coat last. As an alternative to the vest (waistcoat), a correctly worn cummerbund goes over the shirt and trousers, and under the coat. When worn correctly, the cummerbund should appear to slim the wearer, as well as making him appear taller. It is, however, a subtle piece of clothing and shouldn't be worn to draw attention to itself. Make sure the fit and the colour are appropriate for your larger outfit after you are fully dressed.