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. Tap the bell icon and the phone will ring. Tap the Close icon (X) on the watch to stop the vibrate and ring.
2) Tap the Back 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. Tap the bell icon and the phone will ring. Tap the Close icon (X) on the watch to stop the vibrate and ring.