To redeem a Blizzard gift card, you can follow these steps:

Go to the Blizzard website.
Log in to your account.
Click on your account name in the upper right corner and go to Account Settings.
Select Payment Methods.
Click on Redeem a Code.
Enter the 19-digit code that you received with your gift card.
Click on Redeem Code.
You can also redeem your gift card by going directly to Blizzard.com/code and entering the 19-digit code that you received with your gift card.