Basically, you're taking a screenshot of the victim's desktop and using that image as their desktop wallpaper. So what look like actual files are really just images, including the Dock. To complete the effect you need to switch the Dock preferences to Hiding On.
For many of you that will be enough to get going, but if you need any more detail just follow the steps ahead.
Step 1: Taking a Desktop Screenshot
Step 2: Setting the Screenshot as Wallpaper
Step 3: Setting Dock to Hiding On
Your Prank is now complete!!
Step 4: Reversing the Prank
Hope you've enjoyed this Prank Instructable.