For this prank you need a flat bladed screwdriver. You can also use scissors.
Take the screwdriver, a house key or a pair of scissors and pry off the 1, 2, 3, and the 7, 8 & 9 keys off the 10 key keypad on your intended victim's keyboard. That would be the number keys on the right side of the keyboard. They come off fairly easily once you get the hang of it.
Swap them so that the 1, 2 & 3 are on top and the 7, 8 & 9 are on the bottom.
This works best with hunt and peck typists.
However, this prank really throws people because most of us are used to the 123 number layout we see on phones. With a computer, the keypad is the opposite. So right off the bat, visually it kind of looks correct and ads to the confusion.