Instructions may vary according to device or software version and may require some practice.

Begin by getting whatever you want to take a screenshot of onto the screen of your Android device. This could mean browsing to a particular website or simply getting to the right screen in one of your apps.
   

For most devices running Android 4.0 or higher (try this method first):

Press the Power/Lock button and the Volume down button at the same time and hold them for 1-2 seconds.
For devices with physical home buttons:

Press the Power/Lock button and the Home button at the same time and hold them for 1-2 seconds.

For devices without physical home buttons (i.e. HTC):

Hold down the Power/Lock button first and quickly tap the capacitive Home button.

For Samsung devices supporting hand motions:
  1. Open Settings > Motions.
  2. Scroll down and find "Hand Motions"
  3. Activate "Palm Swipe to Capture".
  4. Close the menu and swipe the screen with the side of your hand from left to right or vice versa.
When the capture was successful, your screen will flash and you'll get a notification at the top of your screen. You may also hear a camera shutter sound.

To view your new screenshot, swipe down from the top and tap on the notification. You can also access your screenshot from your Gallery app.