STEP 1: Activate Office 365
Purchase and activate an Office 365 Home Premium annual or Office 365 University subscription between 18 July 2013 and 28 September 2013.
STEP 2: Get your Xbox code
Get your code by October 31, 2013 by signing in with the Microsoft account email and password you used to set up your Office 365 subscription.
STEP 3: Redeem your Xbox code
Redeem your code at Xbox.com or on your Xbox 360 console to begin your 12-month Xbox Live Gold membership.