1.   As it cooks, occasionally skim the scum from the top with a large spoon.

2.   Add the bonito flakes and bring to a simmer over medium heat, skimming any scum that forms on the surface.

3.   Bring to a simmer and skim the scum that comes to the top.

4.   Bring to a boil, skimming any scum that may form.

5.   Skim any scum that forms on the surface.