Fillet a catfish just like you do a crappie, bass or any other species that you fillet. Just have to be a little more careful when you fillet the skin off.
On crappie and bass, I start further forward and cut through the rib cage on the first cut, then fillet off the skin, then cut the ribs out.
On catfish, I stay out of the rib cage. Catfish ribs are thicker and harder to cut through, and the stuff inside of the catfish ribcage is much messier.