1. First click "Gift" Colum and click $10 iTunes Gift Card
2. Click Okay
3. After click Okay you will have iTune Gift Card Voucher Code, copy the code and open your App Store.
Click Redeem like picture below :
4. Input your iTunes Gift Card Voucher Code in the mark like picture below :
5. After input iTunes Gift Card Voucher Code next click Redeem at upper right.
6. Now you can see your balance in the bottom left in your Apple ID $44.74
7. Log into game Clash of Clans and go to Shop. Look my first gems before buying : 2456 gems
8. Now, i buy 2500 gems.
9. Sign in to iTunes Store and click buy.
10. My gems now is "4956 Gems" ^_^
11. Back to my Apple ID, see my balance now.
12. Later there will be a report from an email about purchasing gems.