how do I pay my monthly installments

The first payment doesn't happen quickly. When it is time to make a payment, you'll see the Apple Card go from white to mixed colors in the Apple Wallet. When a payment is due, you'll receive an email notification that a bill is ready. Payments will be due on the first of each month, once bills start appearing. You'll set up payment from your banking account inside the Apple Card in the wallet. Since today is June 1st, you clearly don't have to make a payment yet.

Did you use an Apple Card, or did you use some other method? Did you buy directly from the Apple Store or online Apple Store? You would have needed to submit a credit card when you made the order. Your credit card would be billed and you would pay your credit card bill when that comes in.

You're very welcome.