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

How can I get the Guardin' Gnome achievement in Left 4 Dead 2 on Steam?

The text of the achievement is the following:

Rescue Gnome Chompski from the Carnival.

So I suppose it has to be done in the Dark Carnival campaign, but how?

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.

One little tip:

When you are in the safe room at the end of each map, you need to be holding the Gnome for the map to really finish. Closing the door is not sufficient. Might not be obvious for some (like me :))