Get a white tarp, sheet, blanket or whatever you have that's white. Wrap the car up. I stuffed an old tarp on the top of the car for extra skull bulk. If you have a car with a trunk, perhaps just wrapping the front would be enough, or making both the hood and the trunk into skulls facing opposite directions.
Cut out skull shapes out of black plastic, cloth, garbage bags or use black duct tape. Attach them to the car with outdoor duct tape or another brilliant method I'm sure you will come up with.
I also used this method to dress up a bush I had in the yard. I used a sheet, tied the jaw and teeth line tight with a string to the back bottom side. I straight-pinned cloth shapes to the skull. The teeth are attached to where the string gathers the sheet.
Happy Halloween Decorating!