How do I use an iTunes gift card to buy more iCloud storage

If you've already redeemed the gift card, there is nothing else you need to do. When making a purchase, including iCloud storage, it will deduct from the store credit before it would deduct from your primary payment method.

If you need help redeeming, see: https://support.apple.com/en-us/HT201209.

You cannot use funds from a gift card to pay for iCloud storage. See https://support.apple.com/en-us/HT201238

”Accepted payment methods for iCloud storage upgrades include credit cards, debit cards, and iTunes Store credit. If you don't have enough iTunes store credit to complete your upgrade, you'll be charged the remaining amount. Apple Store gift cards aren't accepted as payment for upgrading iCloud storage. Learn how to manage the amount of storage you're using.“

Apple Store gift cards may not be accepted, but iTunes gift cards are accepted. I pay for my iCloud storage by redeeming an iTunes gift card. iTunes gift cards are iTunes Store credit.

I think this is simply a semantics thing here.

Yes, sounds right now that you state it like that, sorry.