It's easy to take a screenshot:

1) Press and hold the Power button (which switches the device on and off) and the Volume-down button (which regulates the device volume) together.
2) In 1-2 seconds, you will hear a camera shutter which signifies that a screenshot has been successfully taken.
3) After, the system will automatically save it in folders /Pictures/Screenshots or /Pictures/ScreenCapture.
4) Attach the screenshot to our conversation by pressing the Plus button in the bottom left corner of the screen.

P.S. If there is a Home button on your device, you need to press and hold the Power and the Home buttons.