To make sauce, melt butter in saucepan. Add flour and whisk together. Add milk and whisk, whisk, whisk. Turn heat to medium and let sauce thicken. Add more milk to thin it out or more flour to thicken. Whisk so sauce doesn't clump up.
Add cheese and let melt.
Place pasta in baking dish. Pour cheese sauce over pasta and stir through. Top with any remaining cheese and bake 10-15 minutes until top begins to golden.
NOTE: Add the cheese just after whisking the milk in to achieve a creamier texture!