To take a screenshot you have to do is follow these simple steps, fairly soon:
Press and hold the Volume down (-) button and power button at the same time
If the operation is successful, you will see an effect style "flash" on the screen, then the image will be captured and stored in the photo gallery, as well as find yourself a notification from the dropdown menu.
If you do not succeed at first shot do not despair, it's just a matter of practice: the secret is all in the timing.