To temporarily disable Face ID, just press the buttons on both sides of the phone. There is no mention on how long to hold down the buttons, but CBT will let you know as soon as they are available in November.
If you haven’t disabled Face ID, but still want to avoid unlocking your iPhone X with your face, Apple says don’t stare towards the camera of the iPhone X. Apparently Face ID needs you to have your eyes opened towards the camera for it to work.
