There are corners on the world map (zooming out makes it easier), which you can click on once you've unlocked enough tiles to actually reach the corners.
Each corner represents a number, as follows:
Southwest = 1
Southeast = 3
Northwest = 7
Northeast = 9
To get this achievement you need to click on the corners in the order 11331791 and then complete a level with this 'mod' active.
This will make the trees grey. Click any corner 8 times to reset.
Source (and more codes):
http://gemcraft.wikia.com/wiki/Four_corner_map_code