I'm sure that one of our Artists here on the forum can give you the exact way that they do it or more of a profesonal way to go about it, but I can tell you what I would try doing if it were me...
I've never done this, but again, this is what I would atempt...
First I'd do the hair in 2 differant lengths, I would cut some hair just a little bit shorter than the longest strans that you were planning on ung... Then I'd use some form of a glue that is very workable or perhaps maybe watering the glue down "Slightly" to make it thin and easy to work into the hair. I would guess that the hair would naturally work itself into being stringy...
After all the hair is layed and you style it how you want it, I'd use a blow dryer to dry it.
Then I'd take an Airbrush and use a few colors like white, grey and a very thinned down veron of brown. I would take the white and grey and very throughly give all the layers of hair a slight over-spray to create a dusty look to it... Then I'd add that thinned out veron of brown to darken some of the areas that you may have gotton a little to much white in...
I'm not saying that this is the way to go, but I would feel Very Confident in doing this process for one of my own masks if I felt the urdge to...
The only questions I would ask myself is "What Kind Of Glue"??? My first thought would be your bac Elmer's Glue watered down a little... But, I would want something a little more permenant...
I'm sure someone can tell ya how to do this in a more profesonal Manor, but I hope this helps!
--Ryan