Firstly place a saucepan on the hob; turn the heat up quite high as you will need to melt butter. Next melt the butter, and stir it slowly and make sure you don't leave it alone or it will burn and be no use.
Take your butter off heat and add your flour; now put it on the heat for 1 minute make sure you don't leave the sauce alone keep on stirring and stirring it.
Next take it off the heat again and slowly add the milk and stir using a whisk, this will hopefully make it thin. Then place on the hob, whisking constantly, until it is thick and can coat the back of the spoon.
Add your cheese, as your sauce is hot it will melt the cheese and make it tasty and milky.
When adding the milk, don't add it all at the same time as it will be to thin!