"Hello i'm EXPANDING my window cleaning service in this area, and I wondered if you needed a window cleaner?"
If no hand them a business card or leaflet and ask them to give your number to people they know in the area who require your services.
If yes, always ask that you can walk around the whole property so that you can give them an accurate quote.
Check the property out and then ask them "How often do you require you windows to be cleaned,4,6 or 8 weekly?"
When they tell you the duration they require, you give them the price.
When they accept, arrange a day to clean or clean them on the day if possible.
Don't over complicate it with long speeches and the like theres no need.
Keep it basic and natural conversation will flow.