How do I use an iTune gift card to but gems on clash of clans

Tap Redeem at the bottom of the App Store page. Sign into your iTunes account. Use the camera on your iPad to record the number on the back of the card or enter manually. The amount of the card should automatically added to your acount. You may have to sign out of your iTunes account. You should now be able to buy the gems from inside of Clash of Clans. Tap the link below and scroll down to How to redeem for the official version. On the screenshot I included where you tap redeem and where the credit should appear. If you're still not sure you can stop by the Apple Store and they'll walk you through the steps.

Redeem and use iTunes Gift Cards and content codes - Apple Support