There are a number of specialized products available specifically for cleaning copper and brass. If you choose to use one of these, follow the directions on the label. If you'd rather make your own, there's a simple recipe: dissolve a teaspoon of salt in a cup of white vinegar, then add enough flour to make a paste. Apply the paste to the surface you want to clean and let it sit for fifteen minutes. Rinse it off with warm water and dry thoroughly with a towel. After cleaning, you might want to apply a protective polish to protect the surface.