To pair an Apple Watch with an iPhone, open the Watch app on your iPhone, tap the "Start Pairing" option, and hold your iPhone's camera over the animation shown on the Watch screen until pairing is complete. You'll then set up Face ID or passcode on the iPhone, agree to terms, and sign in with your Apple ID. Finally, choose which apps to install on the Watch and wait for the sync to finish.
If pairing fails, restart both devices by holding the side button until the power slider appears, swipe to power off, then power back on. Make sure both devices have adequate battery (at least 50%), are close together, and have Bluetooth enabled on the iPhone. If you're pairing an older Watch model, you may need to unpair it first by opening the Watch app, selecting "All Watches," tapping the info icon next to the device, and choosing "Unpair Apple Watch." After unpairing, you can pair the Watch to your new iPhone following the standard pairing steps above. Both devices should be running current software versions for the smoothest pairing experience.