A ceramic diffuser consists of three parts: an urn that houses the tealight; a shallow dish that sits atop the urn; a replaceable tealight.
Place the ceramic oil diffuser on a flat, stable surface.
Slide the unlit tealight into the bottom of the diffuser.
 Pour about 2 tablespoons of tap water into the shallow dish.
Drop in about 8-10 drops of essential oil into the water. Light the tealight.
 
The purpose of the water is to aid in the diffusion of aromatic particles as the heated water turns to vapor. Also, the addition of the water prevents the essential oil from burning at the bottom of the dish. Never leave a lit candle unattended.