Instructions
Turn your Xbox 360 on. Select the "Xbox LIVE Marketplace" option in the 360's main menu to access the marketplace main menu.
Select the "Browse Games" option to bring up a list of Xbox 360 games available for download. While there are several game demos that are available for free, other full games can be purchased with Microsoft Points.
Select the game of your choice from the available list. Select the "Confirm Download" option to begin downloading your game. If you do not have enough Microsoft points to purchase the game, you will be asked to add more. Use your credit or debit card information, follow the prompts and begin downloading your game.
Exit the Xbox LIVE Marketplace after your download has completed. You can now play your game by selecting it in the "Games" section of the Xbox 360 main menu.