In a small bowl, mix your flour, salt and pepper. Dump into a shallow bowl, or plate.
Pour your milk into another shallow bowl or plate.
One fillet at a time, quickly dunk your haddock into the milk so that it's lightly coated, then lay it in the flour first on one side and then the other so that it is completely covered in flour. I make kind of an assembly line so that it's a plate with the plain fish, then the milk, then the flour, then a plate for the flour-coated fish - makes it go very quickly!
Heat up a frying pan on med-high and melt the butter. Add your fillets, and cook for about 3-5 minutes on each side. You want the outside to be nice and crispy, but the inside soft and flaky. Try not to overcook it, which will make it slightly tougher (but still very delicious!)