An elk bugle is nothing more than a foot-and-a-half-long piece of tubing combined with a reed. Some models combine a tube call, similar to a turkey tube call, and a sounding chamber or tube, others use a diaphragm call and a sound chamber or a flexible plastic reed and a sounding chamber.
As air passes over the reed, the reed vibrates and makes a sound. A bull elk bugle starts out with a high-pitched squeal, called the bugle, concluded with three high-to-low pitched grunts. Bull elk use this call to attract an elk cow and to claim their territory. Many call companies make cassette tapes of elk calls. Listen to a tape and then try to make the sound.
Using an elk call takes a lot of practice, so be patient and practice hard, and you'll be calling like a pro in no time.