Instructions
Chatting
Start the "Rock Band" game of your choice and turn on the instrument controller you want to play with.
Attach your Xbox Live headset to the audio input port on your instrument. You can find this port on the bottom edge of most guitar controllers or on the main panel of drum sets. Depending on the age of your headset, it may need an adapter. Harmonix includes adapters with some editions of the game, but you can also use any 2.5mm to 3.5mm audio adapter.
Connect to Xbox Live by entering the online multiplayer area of the game from the main menu. Once you connect to an online game, your headset will allow you to talk to the other connected players. Be aware that many players do not connect headsets and so may not reply to you.
Singing
Start up a "Rock Band" game and turn on a regular Xbox controller.
Attach your headset to the port on the bottom of the controller. Turn the volume dial on the headset all the way up.
Enter the "Settings" area and check your microphone options. Boost the microphone input level to make up for the weakness of the headset mic compared to a standard microphone.
Start any game mode from the main menu and press "A" on your controller to join the game as a singer. Sing as you would with a regular microphone.