Here's a small guide on how to take a screenshot:
1) Display anything you want to snap on your screen.
2) Press the Side Button and the Volume Up button at the same time (Side Button + Home for older iOs devices).
3) Quickly release both buttons.

Once it’s done, you will hear a camera shutter, which signifies that a screenshot is successfully taken.

Once a screenshot is captured, it will be automatically saved to your Camera Roll. You can head to your Camera roll and check the screenshot you have just taken.