Keep your eyes open and make sure your face is fully visible and not covered by any object, such as a hat or sunglasses.
To unlock your phone, just turn your screen on and look at the front camera.
If there is a facial recognition error, and you cannot use alternative sign-in methods to recover or reset the phone in any way, your phone will require service. Additional charges may apply, and all the personal data on your phone may be deleted. For more info, contact the nearest authorized service facility for your phone, or your phone dealer.