Get a mirror. A big splotch of green, smear it all over your face, cover everything, common spots to miss are your ears and your eyelids.
Next, a little bit of brown, make a few t or x shapes, about an inch long in random places, say one on a check, one on the side of your chin, etc.
Next, a bit of black, not too much, and make a few more t or x shapes, smaller ones then the brown, in a few really high spots, like your forehead, or check bones.
Now smear it all around so it looks like a mess. You're done.