In order to achieve it, please follow the steps below:
Launch Steam and log into your account.
On the bottom left, click ADD A GAME
Select Activate a Product on Steam
Click NEXT then I AGREE
Insert your Product Code and then click NEXT
From your LIBRARY tab you can see all games registered to your Steam account.
You can then select a game and it will bring up a splash page with a large INSTALL button you can click on to install your games. Once your games are installed, the Install button will change to a PLAY button!