It's easy to take a screenshot:

1) Press and hold the Sleep/Wake button.
2) Then press the Home button (or Volume-down button) and release the Sleep/Wake button.
3) If the OS on the device is running on iOS 11 or later, you will see a small image of the screenshot in the bottom left corner of the screen.
To find the screenshot, go to Photos > Albums and click on Screenshots.
Attach the screenshot to the conversation by pressing the Plus button in the bottom left corner of the screen.