You need a professional-quality squeegee, a window wand, and an extension pole if you’ll be doing high windows. The basic process is simple — apply the cleaning solution with the window wand, them pull all the dirt and water off with the squeegee.
Now the details.
Put a capful of ammonia or 5 drops of liquid dish detergent in 2 gallons of water. Resist the urge to use too much detergent; that is what causes streaking.
Dip your window scrubbing wand or a sponge 3/4 of an inch into the solution, picking up just enough water to wet the window without flooding it. Wet the entire window, then go back over it once to loosen any stubborn soil. Last, go all around the window against the frame with the scrubber to pick up any dirt you have pushed against the frame.
Dampen the squeegee blade before you start, and wipe it with a damp cloth between strokes. A dry blade will skip and jump on the window instead of gliding smoothly.
Tilt the squeegee at an angle so that only about an inch of the rubber blade presses lightly against the top of the window glass. Then pull the squeegee across the window horizontally. This will leave a 1-inch dry strip across the top of the window. By squeegeeing across the top first, you eliminate drips running down.
Place the squeegee close to the frame in the dry area near the top, and pull down to about 3 inches from the bottom of the glass. Continue this way all across the window, overlapping into the clean, dry area with each stroke, and wiping the blade with a damp cloth after each stroke.
Finish with a horizontal stroke across the bottom, and wipe any water off the sill with a damp cloth.
On some windows, it’s easier to cut the water off the frame side as well as the top, and then squeegee the entire pane using horizontal strokes.
Large (picture) windows should be wet and squeegeed half at a time, the top half first.
Finally, if you’re doing both the inside and outside the window, you may want to squeegee horizontally on one side and vertically on the other, so you can tell whether any streaks are inside or out.