Activation through the Rockstar Games Launcher application
In order to redeem games on your Rockstar Social Club account, open your Rockstar Games launcher (you can download Rockstar Games Launcher here) and follow these steps:
Sign in with your existing account or create a new one.
Press on the icon on the top right side and then click the REDEEM CODE button which is displayed in the picture below.
The Rockstar Code activation window will pop up. Simply paste your game code into the designated area and click the CHECK button.
Activation through the website
Register or sign in to a Rockstar Social Club account (you can find the account page here) and follow these steps:
Click on the Use Rockstar Code button of any product (the same activation window pops up for every product).
The Rockstar Code activation window will pop up. Simply paste your game code into the designated area and press the Activate button.