This was done on Mac OS 10.6.6 – there may be other versions of Mac OS that are slightly different. However, chances are good that this will work for you!
Here is the video on how to take the screen capture:
In case you were wondering what the object of the screen capture was, this is a book about gum disease that you can find on Amazon or other book seller sites. It is an excellent book by the way. Disclaimer: I wrote it. 🙂
Shift + Command + 4
Ok, comments or questions below!