How to use a iTunes gift card to buy apps?

I know how to redeem them but to buy the app do I need to do anything in particular or will it still say buy app (0.69) and is there a way to choose between card and gift card? And do I need to buy the app through pc or can it be done through the phone?
If it help I have a iPhone 5

Go to the app store, then scroll to the bottom and you will either see redeem or something to do with that.

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