Double-click the right hand side of your iPhone. If prompted, authenticate with Face ID or enter your passcode to open Apple Wallet. Hold the top of your iPhone near the contactless reader until Done and a checkmark appears on the display.
Double-click the side button of your Apple Watch. Hold the display of your watch near the contactless reader until you feel a gentle tap and hear a beep.
Tap the Apple Pay button or choose Apple Pay as your payment method, then use Face ID or Touch ID to confirm the payment. To pay in Safari on Mac, select Apple Pay then confirm the payment on your connected iPhone or Apple Watch.