Am world's lousiest cook and I've just managed to master this (thanks to Annabel Karmel weaning cheese sauce recipe ). Honestly if i can do it anyone cna.
Nice big clump of butter (15g or so)
Two tbsps (or 15g, whatever, don't think it makes much odds) plain flour
150ml or 5fl oz milk
30g of yellow cheese (cheddar, gruyere, emmental, whatever)
2tbsps parmesan grated
mascarpone if you have it but no worries if not.
Butter in saucepan over low heat, let it melt. Don't burn it!
Stir in flour gradually and keep it on flame for about a minute, will become paste stuff.
Now stir in milk, slowly, bring mixture to the boil and cook for few minutes over low heat - should thicken up and be smooth
Remove from heat, stir in your grated cheese until melted and, well, serve!