While the specifics can vary between different brands and models of air compressor, the following basic guidelines apply to most of them.  Position the air compressor on flat, stable ground within reach of an electrical outlet, and plug in the power cord. Don’t turn on the air compressor yet. Check the oil level. Typically, the oil gauge will be near the motor. Note, however, that many newer air compressors no longer require the addition of oil, as they have sealed systems. These air compressors are often sold as “oil free.”  If the oil level is low, add compressor oil—this oil does not have detergents or additives commonly found in automotive oil—to the oil tank until the oil level reaches the “Full” mark. The oil tank access cap is often found on the top of the air compressor. Make sure the drain valve is switched to the closed position. You’ll find the drain valve near the bottom of the air compressor.  Switch the air compressor on, and let it run until it reaches the pressure capacity. For most air compressors, that will be 100 to 115 pounds per square inch (psi). The pressure gauge is normally on the top of the air compressor.  Set the air control valve—it will be on top of the air compressor—to the recommended maximum psi of the tool you plan on using.  Connect the air hose to your air compressor. Some models have quick-connect fittings, while others require you to screw the hose to the fitting. Make sure the hose is tightly secured. You might need to use an adjustable wrench for this.  Connect the other end of the air hose to your pneumatic tool.  Use your tool as needed. When finished, turn the air compressor off, disconnect the tool, and unplug the air compressor from the electrical outlet. Coleman Cosby, project manager and landscape design specialist at Yardzen, adds, “Prior to storing an air compressor, make sure to empty any remaining air from the tank, and tip it so the release valve is pointed down. This helps release any moisture from the tank.”