How to use an itunes gift card if you already set up a credit card itunes account?

I put my mom's credit card account on my itunes and I purchased some application for about $7.00 just to test if the account is active, But I don't want to use it all the time to purchase stuff from the app store. I want to purchase an itunes gift card but I don't know how to use it on my itunes. Can you help me guys, I would appreciate it. thank you
How to use an itunes gift card if you already set up a credit card itunes account?
On the iTunes store homepage, click redeem in the right-hand column.
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