Press the power button on the top right of an iPhone X in tandem with the volume up button at the left top of the phone to take a screen grab.

I figured this out today in absence of the "big button" which is now gone from the modern design. I'm pretty proud of myself. If you hold these buttons too long it will power the phone off, but not before prompting you to use a slider to do so. The power button by itself turns the phone back on but it doesn't want to work until a few seconds pass after a power down.