How do I use my Apple account balance to pay my iCloud subscription ?

It should automatically unless it is a subscription that you can't pay for with balance (Apple doesn't provide a list of which one can't use balance).

Based upon: How apps, content, and subscriptions from Apple are billed - Apple Support

Which payment method is charged?

When you buy items or pay for subscriptions, Apple automatically attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance, you don't have enough to cover the full amount, or an item cannot be paid for with Apple ID balance, Apple attempts to charge your primary payment method (typically a credit or debit card, but varies according to country). The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.

Some purchases aren’t charged to your Apple ID balance

- If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method.

- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

- Some subscriptions might not be charged to your Apple ID balance.

Click on this link to review 'Payment methods that you can use with your Apple ID' --> Payment methods that you can use with your Apple ID - Apple Support - Individual brands of cards are not listed so you will just have to try one and see if it is accepted.