1.See you on http://www.origin.com/
2.Click "Download Origin"
3.Once the client is installed and has started, log in with your username / password (create one if needed).
4.Once logged in, click the small options icon in the top right corner.
5.Select "Enter a product code..."
6.A window will prompt you to enter the code you purchased on Instant Gaming.
7.Origin will authenticate your code after a few minutes.
8.The game is now visible in your list.
9.Click it to start the download.
If you require more specific help, or if you have any questions, don't hesitate to Contact us.