The Galileo barometer is based on a single piece of glass and a fluid, usually distilled water. The glass has an area dedicated to housing a reservoir of fluid and a thin, upward-facing spout that is open to the atmosphere. The water is often dyed so it can be seen from a distance.
To fill the reservoir, use a syringe to overcome the vacuum seal created at the point where the spout connects with the reservoir. The device is usually turned upside down and filled one syringe-full at a time until the fluid is approximately halfway up the spout. There must be room for the fluid level to fluctuate in both directions as the atmospheric pressure changes.
When a low-pressure weather system approaches, the weight of the air pressing on the open end of the spout decreases, allowing the water level to rise. Conversely, when a high-pressure system moves in, it will push the water level down. A scale can be printed on the spout corresponding to the inches of mercury, normally between 28 and 32 inches.