1.Go to 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 on your library on the left menu.
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.