Don't overthink it, just go and do it. Don't feel embarrassed, there's no need. Knock knock, "Good afternoon, would you like a window cleaner?" And take it from there. If they say, "No", leave 'em a card. If they say, "I already have one!" leave them a card with a "Can I just give you this in case you need another in the future?"
If you're lucky, you'll drag an attractive looking woman out of the shower and she'll answer dripping wet and just wrapped in a towel. That happened to me once, but she didn't want a window cleaner or anything else.
I've not door knocked in a few years, but I've read here to do it with an attitude that you have this brilliant service and you're doing your prospective clients a favour by letting them know. I really liked that.