iPhone X and iPhone 8 require different button combos than previous iPhones:
Quickly click and release the Volume Up button.Quickly click and release the Volume Down button.
Press and hold the Side button (sleep/wake, on/off).
So does iPhone 7:
Press and hold down the On/Off button on the right sideWhile continuing to hold the On/Off button, press and hold the volume downbutton on the left side of your iPhone.
Hold both buttons as the screen turns off, and keep holding them until the screen turns back on and displays the Apple logo.
For iPhone 6 and previous iPhones:
Press and hold down both the Sleep/Wake button and the Home button at the same time.Keep them held down until you see an Apple logo.