Don't use stencils. You'll have real problems with hard edges that will ruin your camo look.
You've got a couple of options to a good camo finish. One is to simply spray freehand, with enough diffusion to give you a nice fade on the edges. Apply a khaki green basecoat, and build the other colours on top as required. Avoid moving your arm in a straight line.
The other way is to again, apply a green basecoat, but this time go for a quick walk for your stencils - an old pine branch, the odd bit of fallen foilage, all make great disposable masks for you to spray over that will give you a nice camo effect.
Hope that's of some help!