Since the iPhone X no longer has the home button, here's a quick step by step to taking a screenshot. Open the app or get to the screen you want to capture. When you are ready to capture, press and hold the Side button on the right side of iPhone X. At the same time click the Volume Up button. You should now see the phone's screen flash white and you'll hear the camera shutter sound (if your sound is enabled). Now check Photos and your new screenshot should be there!