Sprinkle the bottom of the oiled pan with cornbread mix.
You may choose to dip your (cleaned and dried) catfish fillets in milk before proceeding; this will make the whole thing a bit moister and help more cornbread cling to the fish. I usually skip this step because we rarely have milk in the house, and the fish drops liquid during the cooking process.
So: take your catfish fillet, milk-dipped or not, and dip it flat-side down into the cornbread mix. Make sure the flat side is fully covered, then lay it flat side down in the pan.