Assuming you have a standard rear window wiper and washer system, it is actually quite simple to fill. You will need to locate the reservoir, which is usually located in the engine compartment near the windshield fluid reservoir. Once you have found it, remove the cap and inspect the fluid level. If it is low, simply add more fluid until it reaches the Full line on the side of the reservoir. It is important to use windshield washer fluid and not plain water, as water can freeze in cold weather and damage your car's wiper system.