This method works (pinched off here years ago so sorry to that person for not mentioning them)
Start with a Pyrex jug/bowl that can go in the microwave. It needs to
be quite big (0.5 litre is fine) as the sauce may rise in the jug as
it cooks.
300ml milk (put this in first)
25g flour
25g butter
Give it a very quick whisk before you start.
Then 1 minute in the microwave on high, whisk and put back in.
Repeat that step until it's had 4 minutes in the microwave in total -
at that point, it will have thickened.
Then stir in 50g of grated cheese
.even easier cheese sauce,mix a tablespoon of cornflour to a paste with cold water and a little mustard powder salt and pepper, bring saucepan of milk to the boil and add cornflour mix stirring well until it thickens, turn off heat, add grated cheese and stir easy peasy cheese sauce.