Instructions
Acquire a headset that is compatible with an Xbox 360. Because Microsoft wants you to use its equipment, you cannot simply use any headset to sing on "Rock Band."
Connect the headset to one of the controllers. For "Rock Band," you need one controller activated for the singer, so simply hook the headset cord into the jack located on your Xbox 360 controller.
Sign in as two separate members of the band. One member must be the singer, which is controlled by the headset controller, and the other can be the guitar, drum or bass player.
Put the controller in your pocket or in a place where it will not be jostled during a song. When you start the song, the microphone on the headset will automatically be set up as the microphone for "Rock Band." Simply sing into the headset during the song.