Get some of our Clean Detail or Fully Slick detailing sprays and spray a lubricating coat on the glass, ready for the clay products to be used on it. Do about a quarter section of the windscreen at a time. You will find the clay bar does not work too well on a dry surface, so be sure it's ample wet! Rub the clay towel or bar on the wet surface, use a circular pattern over small sections of the screen at a time till you feel (and hear on real dirty glass) it's perfectly smooth. Wipe the screen clean with another dry microfibre cloth Use some Naked Glass or the Clean Detail to make sure it's clean again, and allow a minute or so for it to dry before the next process.