If you like buying specialist window-cleaning sprays, save these for cleaning windows from the inside. If you want to know how to clean a window from outside, Ray Brosnan of Brosnan Property Solutions, says don't bother with "fancy chemicals or detergents." He notes that all you really need is a good squeegee, dishwashing soap, and warm water.
When It comes to the technique, Brosnan recommends "the S-Shape method," which entails covering the whole pane in an S motion, making "sure to not take the blade of your squeegee when cleaning the glass or you’ll be left with water marks."