Here's a quick step-by-step guide:
Step 1: Locate the content you want to shoot on your screen.
Step 2: Then tap the screen twice using one knuckle as quickly as you can.
Step 3: You should hear a camera click sound and see a brief animation indicating that a screenshot has been taken.
Step 4: To view your screenshot, open the Gallery app and look for the "Screenshots" folder.
So try it next time you need to capture something from your screen.