Launch the Steam app on your laptop or PC and make sure you are logged into the account you want to redeem the code in.
Go to the very top menu in the left-hand corner of the screen and click on the Games header.
From the drop-down menu presented, click on Activate a Product on Steam.
You will be presented with a new text box. Click on Next.
Click on I Agree.
You can now enter your product code. It will be a 16-digit code and once it has been accepted, you will be able to download the product.