In a small saucepan, over medium heat, melt your butter and pour in your milk. While the milk is still cold, whisk in the flour.
When the milk begins to warm, slowly add in your sliced cheese. (I tear the slices in half and add them one at a time. It could be overkill, but I can't stand the thoughts of stinky, scorched cheese.) DO NOT STOP STIRRING, it will scorch if you stop stirring!
Once all the slices are melted, then you can add 1 1/2 cups of shredded cheddar, again, do this slowly.
The sauce will thicken when it cools, but if it looks too thin you may add the other 1/2 cup of shredded cheddar. It just depends on how thick you like your cheese sauce. I add the salt last, and I add it to taste.