Opening and locking the screen on a Samsung Galaxy S2 is quite easy.
Lock Screen
To lock the screen you press the power button on the right side of the phone.
Unlock Screen
To unlock you do exactly the same as unlocking the screen by pressing the power button. You can also unlock by pressing the main home key on the phone.
If you have security pattern lock enabled then after you have clicked the power button you then draw your pattern to reveal the screen.