Instructions
Log in to your Gold Xbox Live account.
Launch the "Guide" menu by pressing the Xbox button on your controller.
Scroll to "Settings" from the resulting menu then choose "Account Management." The "Guide" menu will close and a new "Account Management" menu will appear.
Press "A" to choose "Memberships" from the "Account Management" menu.
Choose your Gold membership from the resulting "Memberships" screen. A pop-up menu will appear.
Select "Change payment options." A new menu will appear where you can change your Xbox Live Gold account to use a prepaid code.
Press "A" to select "Redeem Code" from the resulting "Payment Options" menu.
Enter your 25-digit prepaid card's code, then press "Start." The Xbox 360 will verify the code over Xbox Live, then attribute the card's monetary value to your Xbox Live Gold account.