I make mine in a saucepan on the stovetop:
1 lb elbow macaroni
1/4 cup butter
1/4 cup flour
1/2 tsp salt
2 cups warm milk
1/2 lb shredded cheddar cheese
Cook macaroni.
Melt butter in saucepan. Stir in flour over medium heat and cook to a roux (3-5 minutes).
Stir in salt and a dash of pepper.
Slowly stir in milk and cook until bubbly.
Stir in cheese a little at a time until melted.
Drain macaroni and stir into cheese sauce.