In any case, I believe you simply lay it over your helmet and strap it on with elastic or whatever. Also, I've only really seen these pre-made ones in use in desert environments (i.e. Iraq, Afghanistan) while I've sometimes seen Marines wearing covers with burlap sewed on much like a ghille suit:
Uhhhhh, don't you know sir (looks around nervously)
At long range a helmet is easy to see on someones head or sticking out above a ledge or building edge. With the camo, as has been mentioned already, this shape is removed. Most of the time it's just a piece of camo netting cut into pieces and it is the decision of the individual commander if his troops wear it or not. There are some things not totally governed by red tape!!
Sorry guys, I wasn't really looking for a history of foliage on helmets, which we all know has been going on for a long time; I wanted to research the current state of affairs specifically relating to the USMC in Iraq, hopefully with some pictures and even more hopefully with some comments from a serving Marine.
the 82nd has been putting camo of some kind or another...canvas, camo net, burlap, etc...on its helmets for 60 years now..other'n that, the only division that has anything famous about its helmets that i can think of is 3d ID, they have always had their unit patch on there