Water, a few drops of oil, and a fine cool mist. An ultrasonic diffuser does not heat the oil, which is the argument for using one: nothing in the oil is altered on its way into the room.

This is the compact one, a small round unit in plain white with coloured light settings you can leave off entirely if you would rather not have a glowing object in the bedroom. It shuts itself off when the water runs out.

Open the unit, add water and your oils, close it and switch on.

Dolphin Clinic suggest emptying it away from the air vent, which is the one thing people get wrong.

Suits any room and any decor, and it takes any oil in the range.