Step 1: Download and install Origin application on your PC. The link to official download page can be found here.
Step 2: Launch the Origin client. Login to your account if you already have one or register a new account.
Step 3: Select "Origin" from the menu in the top-right left corner. In the dropdown menu select an option "Redeem product code".
Step 4: Enter the cd key that you would like to activate on Origin and press "next"