For the smoothest process, go to https://apps.apple.com/redeem/

Otherwise, go to the App Store app

At the top of the screen, tap either sign-in or your photo if you already are signed in

Click on Redeem Gift Card or Code. If you don't see this, please make sure you're signed in.

Click on "Enter Code Manually", then follow the instructions

There is a limit of $2000 on your Apple Account Balance