Tap Set up Face ID, place your device upright, place your face in front of the device, then tap Get Started, place your face inside the frame and gently move your head to complete the circle. Strangely enough, Face ID will continue to learn your face as time goes on. So, if it doesn't work at a certain angle, don't change the position of the phone and try again. Instead, enter your passcode so that the phone can learn more about the shape and angles of your face.
This also applies to cases of aging or growing a beard. If you suddenly go from having a beard to being clean-shaven, you'll need to enter your password to make the phone take note of the changes to your face. But if you stop being well shaved and let your beard grow little by little, your phone will unlock at any time and it will get used to your face when your beard has completely grown.