In a saucepan, heat up the milk, and bring to a gentle simmer. In another saucepan, melt the butter and immediately add the flour. Lower the heat, and mix until the butter has absorbed all the flour and is the consistency of thick dough. Make sure the butter-flour mixture does not burn or brown.
Remove the saucepan from the heat, and whisk in a couple of ladles of heated milk into the butter-flour mixture. Whisk until the flour has absorbed all of the milk. Put the saucepan back on the burner over medium heat, and whisk in the remainder of the milk. Continue whisking until the sauce thickens and becomes a creamy velvety consistency.