The Payment step is the last one in the checkout. The price is held for a short time while you pay, and the line at the top says how long.
Type the code under Have a coupon? and press Apply. Your codes are under Points and coupons in your account.
The Order summary shows your tickets, any coupon or wallet credit, and the total.
Choose a payment method. Its line shows the booking price, any payment fee, and the amount in your card's currency if that is different.
Press Pay. Your bank asks you to confirm with 3D Secure, and your voucher is issued as soon as the payment clears.
Tip. A coupon is only used up once a payment goes through. If your card is declined, the coupon is still yours.