Take a small saucepan and put it on low heat with the butter. Melt the butter and now add the flour.
Fry the flour for a while (make sure it doesn't burn!) so that the flour is cooked.
Now add the milk little by little. If the mixture is still thick, use a wooden spoon to stir. If it becomes thinner you can switch to a whisk. This is easier to remove lumps.
Continue until all the milk is used up or the sauce has reached the desired thickness. Season to taste with salt and pepper.