Clean the glass that has the scratch. Any dirt or other material on the glass when you polish it may further scratch the glass, making the situation worse.
Squirt a small amount of whitening toothpaste onto the glass. Using a soft cloth, begin rubbing the paste into the area of the glass that has the scratch using a circular motion.
After a short time, rinse off the paste and check to see if the scratch is gone. If not, try again. If the toothpaste doesn't seem to be working, try the slightly more abrasive paste made by mixing equal amounts of iron oxide, water and glycerin.