On the screen you want to capture, place the edge of your palm along one edge of the screen (lengthwise, position your hand like you are making a karate chop) and swipe your palm edge across the width of the screen (maintaining your hand in the vertical "chop" position and maintaining it in contact with the screen). You can do this either left to right or right to left, doesn't matter. If successful, you will capture the screen and your default gallery app will appear asking whether you want to keep or discard the capture. I find it finicky and prefer to just use the simultaneous power + home button press method to screen capture.