The first is through the Android notification menu, to display the window icon "Capture" is, if you clicked will perform a screenshot of the window below the bar. If for some reason you do not see it, you should move laterally the list of icons to the right, as the screen capture is the first on the left as seen in the image below.
You may wonder why the photo was taken with another phone rather than screen capture, the reason is that I did not get how to make a screenshot to the drop-down Android notification bar.
But if instead we open the full menu of options for quick configuration of the device, that itself it could catch, and in this if they can see clearly how the icon to capture screen (the first top left):
But sometimes often happens that we are in a game or an application of occupying 100% of the screen and you can not lower the bar notifications for capture and that's where comes the second way (which I consider simpler and easy to use).
To do this you need only press while the power button and the volume down for a couple of seconds (ie, the top and the bottom of which is on the right), you know that the capture was done because you will see in the screen effect and you saved the indicator that is being saved capture as shown in the following image appears at the top: