First, on your iPhone make sure Bluetooth is on and you’re connected to Wi-Fi, then open the Watch app. Keep the Watch unlocked and on your wrist near the iPhone, and when the dialog asks if you want to use your Watch with the new iPhone, tap Continue or tap “Start Pairing.” Align your iPhone’s camera with the animation on the Watch screen (or choose “Pair Manually”), then follow the onscreen prompts: enter your passcode, pick whether to set up as new or restore from backup, sign in with your Apple ID, and choose your preferred settings. Keep both devices close until syncing finishes and you’re all set.