Since the beginning of iOS, the process for taking a screenshot involved pressing the sleep/wake button and the home button at the same time. The screen would flash, emit a camera shutter sound, and the screenshot would be saved to your camera roll.
With the reinvention of iOS devices that lack a home button, starting with the iPhone X, and now the new iPad Pro, the mundane task of taking a screenshot has changed.
To take a screenshot on the new iPad Pro, press the sleep/wake button -- it's the button on top of the iPad Pro -- and the volume-up button at the same time.
You don't need to hold the buttons in for an extended amount of time. Just make sure you press them at the same time, and the familiarity of a screen flash and camera shutter sound will occur.