First thing, remove everything on window surface and window sill. Apply Seventh Generation Glass Cleaner to window. Use a microfiber cloth or a 100 percent cotton shirt or special glass microfiber, clean from left to right, top to bottom.
The reason you need 100 percent cotton or glass microfiber is because other cloths leave dust behind that make the glass dirty.
Newspapers and magazines sometime will leave marks on glass or a toxic residue in your house.
Never leave windows wet.
Vinegar (3:1) can be used if you don't have glass cleaner.