Nice flat light pressure on it should be fine, use a NEW blade, if you have a lot to do change the blade after a little bit to make sure you keep a sharp edge and hold the blade at an angle to the glass... I suppose a little soapy water would not hurt.
I have never had one scratch the glass on a car or house windows.
If you are concerned about it scratching, find a piece of glass and spray a little paint on it, let it dry and scrape it off. Look it over to see if it satisfies you before you hit your car windows.
