You can take a screenshot using your Android device with Android
4.0 (Ice Cream Sandwich) or later.
Press the Power and Volume Down buttons at the same time.
Note: Some devices need you to hold the Power and Volume Down buttons until the screen flashes. If pressing the buttons doesn't work, try holding them for 1 to 2 seconds.
Note: If pressing or holding those buttons doesn't work, then your device manufacturer may have another method. Check their website for more information.
Your screenshot can be found in the Screenshots album in the Photos or Gallery app.