You’ve bought a new Apple Watch Series 4 and you need to set it up with your iPhone. Here’s how:
Turn on your Apple Watch by pressing and holding the side button until you see the Apple logo.
Hold your Apple Watch close to your iPhone. Wait for the “Use your iPhone to set up this Apple Watch” message to appear on your iPhone, then tap Continue. If you don’t see this message, open the Apple Watch app, then tap Start Pairing. Keep your Apple Watch and iPhone close together until you complete these steps.
Center the watch face in the viewfinder on your iPhone. Wait for a message to say that your Apple Watch is paired. If you can’t use the camera, tap Pair Apple Watch Manually, then follow the steps that appear.
If this is your first Apple Watch, tap Set Up Apple Watch. Otherwise, choose a backup. If asked, update your Apple Watch to the latest version of watchOS. Read the Terms and Conditions and tap Agree, then tap Agree again to continue. (Yep, you have to do it twice.)
If asked, enter your Apple ID password. If you aren’t asked, you can sign in later from the Apple Watch app: Tap General > Apple ID, then sign in. Certain features that require a cellular phone number won’t work on cellular models of Apple Watch unless you sign in to iCloud.
If Find My iPhone isn’t set up on your iPhone, you’ll be asked to turn on Activation Lock. If you see an Activation Lock screen, your Apple Watch is already linked to an Apple ID. You need to enter the email address and password for that Apple ID to continue set up. If your Apple Watch was previously owned, you might need to contact the previous owner to remove Activation Lock.
Now choose your settings. Your Apple Watch shows you which settings it shares with your iPhone. If you turned on Find my iPhone, Location Services, Wi-Fi Calling, and Diagnostics for your iPhone, these settings automatically turn on for your Apple Watch. You can choose to use other settings, like Route Tracking and Siri. If Siri isn’t already set up on your iPhone, it will turn on after you choose this option.
Now create a passcode. (You don’t have to, but you’ll need a passcode to, for instance, use Apple Pay.) On your iPhone, tap Create a Passcode or Add a Long Passcode, then switch to your Apple Watch to enter your new code. To skip, tap Don’t Add Passcode. You’ll also be asked to set up Apple Pay by adding a card.
Then you’ll be walked you through setting up features, like SOS and Activity. On cellular models of Apple Watch, you can also set up cellular.
Now you have to wait for your devices to sync. Keep your devices close together until you hear a chime and feel a gentle tap from your Apple Watch, then press the Digital Crown.
Now you’re ready to go!
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)