Melt a knob of butter in a saucepan, then add a dessertspoon of flour and stir until its slightly browning. Quantities vary, so for two people I'd use maybe two spoons of flour and adjust the butter (I use oil) so that the oil and butter are not too runny but not too dry.
Add cold milk, maybe half to three quarters pint of milk - just chuck it in all at once. Stir and heat gently until it thickens. If too thick when it reaches simmering point, then add more milk.
When it's simmered for a minute or two add grated cheese and stir till it's all melted and your sauce is smooth.
I never measure quantities; you need to either find a recipe on line with quantities, or use your own judgment and experience. In any case, it depends on how thick you want your sauce.
Don't worry about pre-heating the milk. You must stir all the time up to stop it thickening at the bottom where it touches the pan.
If you like, you can add a bit of stock powder or salt, pepper or whatever.