You must have 1.1.3 or higher firmware installed on your iPhone to enable this feature.
Make sure you have added installer.iClarified.com as an installer source.
Press to launch Installer from your SpringBoard.
Press to select the Install tab at the bottom of the screen.
Press to select iClarified from the list of Categories.
Press to select Screenshot Enabler from the list of Packages.
Press the Install button at the top right of the screen.
Press the large red Install button that appears.
You will be notified that you must be on the 1.1.3 or newer firmware. Press the large OK button.
A notifications will appear telling you how to use the new screenshot feature. Press the large OK button.
You will be returned to the Categories menu once installation is complete. Press the Home button to return to your SpringBoard.
Now to take a screenshot simply hold down the Home button and toggle the Silent Switch. You will notice the screen flash white.
To view the screenshot you just took, press the Photos icon on your SpringBoard.
Press to select Camera Roll from the list of Photo Albums.
Press to select the small thumbnail of your screenshot.
Now you can see your high quality iPhone screenshot. You can easily sync this to your computer just as you would with any other photo!