Instructions
Connect a keyboard to your console. Plug the USB cable into an available USB port on the Xbox 360. Alternatively, connect the Xbox 360 chatpad to your controller. Connect the male plug on the chatpad to the expansion port on the bottom of your controller. If you do not have these devices, you can use your Xbox 360 controller.
Log in to your Xbox Live profile. Press the Xbox "Guide" button. Go to your profile tab. Select "Messages" and press "A." A Community window opens.
Select "Create New," then press "A." This opens the Create New window. Select "Message" and press "A." A list of gamertags populates in the Select Gamertag list. This list consists of all friends, pending friends and acquaintances you have met in recent games. Select a gamertag and press the "A" button.
Select "Add Text" and press "A." This opens an Enter your Message window. Type a message if you are using a keyboard. If using a chatpad, use the QWERTY keys to create a message. If you are using a controller, look at the on-screen menu to compose your message.
Press "Enter" on your keyboard, or press "Start" on your controller to complete the message. This takes you to a confirmation screen. Select "Send Message" and press the "A" button.