Cardinals have very diverse eating habits. They normally eat insects, berries, spiders, wild fruits, and weed seeds. Cardinals also prefer perching while they eat.
Cardinals have territorial eating habits. If you want to set up a bird feeder it is important to remember this fact. you should set up your bird feeders on opposite sides of your house, which will allow your house to function as a natural landmark.
Since cardinals are perching feeders, have your bird feeder elevated so that it will feel natural to the birds. Their territoriality is lessened in the winter, so you may choose to only use one feeder during colder weather. Also, use black-oil sunflower seeds as feed.