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!