How do I turn off iPhone X?
I can see three ways to turn off an iPhone X.


Press and hold any Volume key + the side lock key, together. Press and hold these keys for a few seconds and the “slide to power off” option will appear on the screen. 
Go to Settings >> General >> Shut Down.
Using Assistive Touch. Just go to the “AssistiveTouch” menu, then select “Device." Tap and hold the Lock Screen icon. Within seconds this will bring up the “slide to power off” option on your screen.


Source - Mobipicker