Q: Redeeming itunes gift card for in app purchases

Hey Chicago2016,

When you redeem your iTunes Gift Card, it will be billed before your credit card:

Order of iTunes billing methods

 

When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.

 

via: iTunes Store: How iTunes Store purchases are billed

http://support.apple.com/kb/HT5582

Once you've successfully redeemed your code, the credit amount will appear next to your account name in the upper-right corner. Each time you purchase items, iTunes will deduct funds from the credit until it's depleted. If a pending purchase exceeds the credit amount, iTunes will ask you to enter a credit card number. You do not need to re-enter your gift card number to make multiple purchases.

 

via: iTunes Store: How to redeem a code

http://support.apple.com/kb/ht1574

So all you'll have to do is redeem it and the funds will automatically be taken first. That second article, iTunes Store: How to redeem a code, will include detailed steps on redeeming your iTunes Gift Card.

Best,

David