Bring 4L / 8.5pt of water in a pot to a boil. Add chicken and pork bones and boil for 10 minutes. A lot of scum will surface.
Drain and wash the bones under running cold water one by one, removing coagulated blood, guts along the spine of the chicken and other brown dirty bits.
Add the cleaned bones and the rest of the Ramen Broth ingredients, excluding bonito flakes, to a large pot with 4L/8.5pt water, and bring it to a boil.
When scum surfaces, occasionally scoop it off gently using a ladle (note 5). Do not mix the broth with the ladle when removing the scum as it will cause the broth to become cloudy.
After removing the scum 4-5 times, turn down the heat to simmer gently.
While simmering, remove scum a few more times in the beginning if required.
Simmer for 2 hours with a lid on but allowing for slight ventilation. Then add a bag of bonito flakes. Simmer for a couple of minutes.
Turn the heat off. Put the broth through a sieve and collect only the liquid.
Makes about 1.6L / 3.4pt of soup (note 6).