Melt the butter in a pan over medium heat. Once melted, add the flour and whisk together. Whisk for a few minutes whilst it bubbles, to cook out the flour. Then add the milk bit by bit, whisking until smooth between each addition.
Once the milk is fully added, keep stirring until you reach your desired consistency. You want it thick and glossy but not stiff. Season well with salt, pepper and nutmeg.
Notes
If I want a thinner sauce, I tend to add more milk bit by bit at the end until I reach the consistency I'm happy with