How do I use my Apple Account balance?

O.k, go ahead.

Buy something from the iTunes, App or Apple TV stores and it should automatically use your account balance.

If you have an issue your current payment method, it will request you fix that, and verify your payment method before charging your account balance though.

This document outlines what you can buy with your Apple Account [Apple ID] balance --> What you can buy with your Apple Gift Card or Apple Account balance - Apple Support