On the GOG Website:

1. Go to GOG.com. (https://www.gog.com/en/) 
2. Log in to your GOG account (or create one if you don’t have one).
3. Enter your game code in the provided field.
4. Click "Continue" and follow the instructions.
5. Once redeemed, the game will be added to your library.
6. You can now download and install the game from the GOG Galaxy app or directly from the website.

On GOG Galaxy (Optional)

If you prefer using GOG Galaxy: (https://www.gog.com/galaxy)  

1. Open GOG Galaxy (download it here if you don’t have it).
2. Log in to your GOG account.
3. Click on the GOG.com menu in the top left.
4. Select "Redeem Code".
5. Enter your game code and confirm.
6. The game will be added to your library for download.