Sand the shiniest parts of the metal. Move the metal around to see which parts of it really catch the light and reflect it. Rub these areas with fine-grain sandpaper. This is especially important with very modern metals like stainless steel. This will give the metal an older look because antique metal products will generally not have as much shine as newer products.
Give the metal a coat of matte paint gloss. Spray the gloss from at least six inches away in a well-ventilated area. Let dry for several hours (or otherwise, depending on the gloss's instructions) until the coat.
Apply black antiquing paint. Use either a jewelry antique paint or high-gloss acrylic paint. Rub the paint into the cracks and crevices of the metal shape, especially if there are any decorative or filigree areas. Use a damp rag to wipe away the paint from the forward surfaces of the metal, leaving the black in the cracks.
Paint rust powder on the metal. Mix iron oxide dust into polyurethane metal varnish to create a rust "paint" and apply it to areas of your metal. Apply the rust to the edges, in cracks between intersecting or overlapping pieces of metal, or on any area that would be likely to be exposed to more moisture over the life of the metal if this really was an old piece.