This will help you transfer a (CUSTOM) Halo 3 map from one account "on an Xbox 360 hard drive" to another on that same hard drive. One way this technique can be useful is if you made a custom map on one of your non-xbox live accounts but you want to be able to play that map on xbox live with your friends.
Just a little heads up
During the instructions it will often say (Next Image) which means you can pick the next image in line so that you can see everything that I am doing
Step 1: Insert and Login
Insert Halo 3 into your Xbox 360 Console
The Xbox 360 Dashboard should appear on your television soon
Press right on your controller and select "Sign In"
It will bring you to a list of all the accounts on your xbox 360 hard drive
Pick the account that has the Xbox 360 custom map on it that you want to transfer
After you have logged on we can continue to The next step of Loading and Forging