1) Swipe down on the watch face, go to Control Center, and tap the Find Phone icon. The watch will display "Searching..." and the phone will vibrate. And you can tap the ring icon to let the phone ring or disable it. Tap the Close icon (X) on the watch to stop finding the phone.
2) Tap the side button on the watch face to go to the app list and tap More > Find Phone. The watch will display "Searching..." and the phone will vibrate. And you can tap the ring icon to let the phone ring or disable it. Tap the Close icon (X) on the watch to stop finding the phone.