Whether you are using fresh or frozen catfish filets make sure that they are completely thawed. Make sure to remove anything red on the meat by shaving it off with a good sharp knife. Pat dry with a paper towel.
Put oil in a Dutch oven or similar deep cooking pot. Heat it until when you put in the end of a wooden spoon handle and see it bubble around the handle.
While the oil is heating, mix all of the dry ingredients it a flat bottom bowel. Take each filet and press it into the cornmeal mixture. Cover the top surface with mixture and press in firmly. Flip and repeat. Place filet on a plate while you're waiting for the oil to heat. This allows the oil in the fish to better absorb the coating. It help lesson the amount of batter that that falls to the bottom of the pan. (You can make up and add more coating as needed.)
Cook each side about 3-4 minutes until coating is dark yellow not quite brown. When you insert a fork it should be easy to insert and remove. This is a good indicator of doneness.