Dissolve the butter in a pan and add the flour.
Stir until the mixture forms a smooth paste which leaves the sides and base of pan cleanly.
Cook for a minimum of 2 minutes to cook out the taste of the flour.
Pour in approximately one-fifth of the quantity of your chosen fluid and allow to boil without stirring.
Stir until the mixture blends smoothly, beat vigorously and add the remaining quantity of fluid as described, beating well after each addition.