Apply the RainX to the windshield, be careful not to get it on the cars finish.
In small circular motions, much like waxing a car, cover the entire windshield with the RainX. You can think of this as waxing the windshield.
Allow to dry to a light haze and then use a clean rag or paper towel to polish/remove excess. Make sure to remove all of it, otherwise your windshield will look dirty.
Enjoy greatly improved visibility when it rains, especially at night. Typically at night, you wont even know its raining until it really starts coming down.