Thank you for checking out this tutorial. Reading the discussions in this post, seems like there is quite a number of people having troubles to take screenshots with Galaxy S2. To clear any confusions, I summarized some information and added as followings. Hope you could make it too. Fingers crossed.
First, make sure your Galaxy S2 is running Android version 4 or above.
Volume Down + Power Key will work fine.
Press both keys at the same time gently. In about 1 second, screen capture would be done. A notification and an animation would be shown too.
Edited on 12 July.
Press and hold power and volume down button together. This is the way to take screenshot in the new Android system 4.0 Ice Cream Sandwich. After the Samsung Galaxy S2 updated to this new version, it uses this method instead of the way mentioned in my old post.