How to use your Apple ID balance

Set your Apple ID balance as your payment method, see -> Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

See also -> Add money to your Apple ID balance

Add money to your Apple Account balance - Apple Support (IN)

Purchases are normally deducted from your account balance before your payment method is charged. You don't need to do anything to make that happen.