To make in-app purchases using your Apple account balance, you must first ensure that you have sufficient funds in your account. You can check your balance by opening the App Store or iTunes Store app on your device, then scrolling down to the bottom of the screen and tapping on your account name.
Once you have confirmed that you have enough funds in your account, launch the app that you want to make purchases in. When you are prompted to make a purchase, you will have the option to use your Apple account balance to pay for it. Simply select this option and confirm the purchase to complete the transaction.
If you do not see the option to use your account balance, it is possible that the app does not support this payment method. You may need to add a credit or debit card to your account and use it to make purchases instead. Additionally, some apps may have restrictions on which payment methods they accept, so be sure to check the app's settings or support documentation for more information.