Take a 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.