To clean windows, use a solution of a few tablespoons of Dawn dish soap with about 3 gallons of cold water.
Use an Etorre or Sorbo squeegee, if possible, and you should have a few different sizes. Having A 6",10", and a 14" squeegee works well for residential windows. You'll want an 18" or even 24" if you are doing commercial. Any applicator will do fine, but the squeegees should be bought o line from a window cleaning supply company.
As for techniques, unless you know how to "fan" already, stick to straight pulls from side to side.
Start the side pulls from the top, and go until the very bottom, wiping the squeegee rubber after each pull. Finish by wiping the streaks or drips with a CLEAN towel, or paper towels.
If it you are at a house with storm windows, double-hung combinations, you will need to remove the storms and use mats or towels to place the windows on, lean each window against a wall with towels propped between the wall and the window. Clean each side of the window and then put it back into the window tracts only AFTER you are done with all the rest of the windows.
I personally would recommend subbing the work to a professional company if the house has storm windows.