How to get the 'Guardin' Gnome' achievement in Left 4 Dead 2?

There was an eHow article (now archived) detailing how to unlock the achievement, but the basic steps are as follows:

In stage 2 of the Dark Carnival, score 750 points on the shooting game before time runs out. Upon winning, you get Gnome Chompski.

Your party then has to carry Gnome Chompski into the escape vehicle at the end of the campaign. You can drop the Gnome as much as you want, and any player can pick the Gnome up, but you must carry him to the end.