There are many different ways of doing this. It depends on what kind of finish you want to achive.
For a new, fresh camouflage you could use an airbrush.
For a faded finish I use diluted white paint, and brush it on in very thin layers, in down ward strokes.
For a rough, chipped finish, I have used Maskol.
Like so.

For a hastily applied whitewash you can brush it roughly and randomly, but to scale, as if it was done with a brush, rag or something like that.
Cheers
Henk