Instructions
Create a Bungie player profile. Visit the Bungie website (see Resources), left-click on "Sign Up" and enter in your current Windows Live account information to make your profile active.
Sign into your new Bungie profile. Left-click on "Edit Profile" on the left side of the page, then left-click on the "Profile Information" tab.
Scroll down the page until you see the section that reads "Personalize Halo 3." Left-click on the "Show" button across from "Halo 3".
Start a new game of "Halo 3." Join the online matchmaking lobby and look at your call sign. It should be a Bungie symbol.