From the Home screen, tap Settings.
Tap Sign in to your iPhone.
Tap Sign in Manually.
If you already have an Apple ID, follow the steps to set up your account on this device. If you don't have an Apple ID yet, tap Don't have an Apple ID? and if you are attempting to log into another device, tap Use Another Apple Device.
Tap the Email or Phone Number field.
Enter your email address.
Tap continue.
Enter your password.
Tap done.
You may be asked to input a code sent to another iOS device using the same Apple ID, as well as your iPhone's passcode if one has been set up.
Navigate to the Watch app and tap General.
Tap Apple Account.
If you skipped sign in at initial set up, you will need to sign in after this step.
You've completed the steps!
