go to a store, buy a sharpie or some kinda marker you can wash off (crayola, whatever)
a black one and a white one, red if you want blood, other colors if you want other random stuff.
outline your eye sockets, and draw on the features for your face (you're going to be a skeleton, and you're going to love it.)
teeth on the lips (leave a few places blackened-in, missing teeth is a plus), a nice little arc to separate your ears & etc, and some space for your cheeks, which will be a problem to make gray (since you didn't buy gray, you foolish person)
use black to cover up the rest of the spaces, since you don't want to show your skin tone, you want black/white. if you want, you can add on a few little details like a crack in the skull or what have you, but too much can make it look overdone.
if you want to have any blood or extra things, put it on before you blacken everything, try not to cover up white, or leave space when you're coloring the white. blood from the eyes, blood from the ears, etc.
find a black shirt and jeans. if you want, you can draw a crude skeleton/ribcage on the shirt.
other than that, you can always be, y'know, a ghost. just get a dirty old bedsheet and cut two eyeholes out crudely. presto, instant ghost.
if you have a full suit, you can go as a mobster. just remember to have a hat/fedora style hat, as all mobster have them. no exceptions.