For the iPhone X ($899 at Amazon), the home button is gone, and with it, Touch ID. Instead, a fancy new True Depth camera maps your face and uses that map for things like unlocking your phone and using Apple Pay. If you skipped past setting up Face ID during initial setup, or simply want to show a friend or relative how easy it is to set up, here's how to register your face with Apple's latest biometric trick.
Open Settings > Face ID & Passcode > Set Up Face ID > Get Started. You'll need to hold the iPhone X between 10 and 20 inches away from your face.
Follow a couple of prompts, then when you're asked, make sure to move your face around so the True Depth camera can learn your face. You don't need to go in a circle, but it does make the process easier.
The system collects 30,000 points of data during this time to ensure that it can tell you apart from someone else. Scan your face a second time, and then you're set.
Since you're already in the Face ID settings page, take a moment to go through and ensure that it's going to work how you prefer. For example, do you want to have to look at the phone for it to unlock? Then leave Require Attention for Face ID turned on. Attention Aware Features is another interesting use, where the iPhone X will alter its behavior if it knows you're actively looking at the screen.