How do I buy clash of clans gems with my apple id balance?

Hello,

In order to use your apple id balance, first you need to select a payment type, say, a VISA credit card. When a credit card is selected as your payment type(After you enter your card number, and the 3 digit code on the back of your card, etc). Try purcharse again, watch out, don't buy $20 worth of gems if you only have $10 balance.