I use very fine steel wool on the windows, lightly around the edges until I get the look I want. Baby steps... A little watered down brown acrylic paint works well on top of that. Mix in a little black pastel chalk powder to grunge it up a bit. The nice thing about using a water based paint it is will wash off if you don't like the look and want to start over. Add a couple of bullet holes by twisting an exacto knife blade into the glass on the outside and moreso on the inside, since bullets tend to spider the glass a bit as the bullet passes through. Scratch in the spider look with short strokes using the blade. Add some cracks by scoring the glass with the exacto knife. Just some ways I've done it.