Clean the brass with soap and water, then dry it completely
Degrease the surface with rubbing alcohol or acetone
Lightly scuff the brass with fine steel wool or a scouring pad
Apply a tarnishing solution such as liver of sulfur, ammonia fumes, or a commercial brass patina
Place the brass in a sealed container with a small dish of ammonia and salt for a darker patina
Spray or wipe the solution evenly over the brass surface
Let the brass sit until the desired tarnish develops
Rinse the brass with clean water to stop the reaction
Dry the brass thoroughly
Seal the finish with wax, lacquer, or clear coat if you want to slow further tarnishing