Step 1: Head over to Epicgames.com and sign in to your Epic account or register a new account.
Step 2: Once logged in, follow this link to access product activation page. Enter your product key and press "redeem".
Step 1: Download and install Epic Games Launcher on your PC. The link to official download page can be found here.
Step 2: Once logged in, click on your profile name in the bottom-left corner and select "redeem code" from the menu. On the following page you will be able to enter your cd key and redeem the code.