Here's how I make it. Put they flour and some milk in a airtight container. Shake. Pour into a pyrex jug and fill to half pint with milk and put a knob of butter in. Microwave for 2 mins, stir, microwave for another 2 mins, checking that it doesn't bubble over. Remove from microwave, stir add seasoning (salt, pepper, cayene, mustard powder) then add grated cheese. The heat of the white sauce will melt the cheese.
Been doing it like that for years and never had a lump or burnt a pan!!
Good luck!