Taking a screenshot using the Samsung Galaxy S2 works the same way as you do with an iPhone.
Press the home button and the power button together and you should hear a sound of a screenshot being taken. The screen will flash and you should see a text that shows it has been captured and saved.
The image above is a sample screenshot taken from my Samsung Galaxy S2 smart phone.
Note: I had been reading through comments and it is true that different o.s. versions and roms like CyanogenMod have different ways to get a screenshot than when I first had my Galaxy S2.
Someone may have already pointed this out but just in case, you may want to try pressing them sequentially but without any delays:
Press volume down button then press power button. This is what I do with my Cyanogen Mod custom ROM on Jeally Bean 4.2.