Peppercorns and/or herbs (optional and strongly discouraged the first time)
Rinse the chicken bones or parts under running water. If using whole chicken or chicken parts, remove and discard any excess fat and, if desired, skin.
Place the chicken in a tall, narrow, metal stock pot and add enough cold water to cover by about an inch. Place over high heat and bring to a gentle boil. Boil gently for about 3 minutes. Quite a lot of scum and fat should appear on the surface. Gently stir the chicken just once to allow any trapped scum to rise to the surface.
Place a colander in the sink. Dump the contents of the pot into the colander and allow the water and scum to go down the drain. This removes the protein and blood that form the scum and foam (but not the chicken flavor). Rinse the chicken and wipe the pot clean.
Return the chicken to the pot, add 16 cups cold water and the onion, carrot and salt. If adding peppercorns and/or herbs, you may add them now for a strong flavor. The chicken should be submerged. If it is not, try to rearrange the chicken before adding more water. If absolutely necessary, add a little extra water. Place over medium-high heat and bring to a simmer. Do not boil. Reduce the heat to medium or medium-low and simmer gently. A stray bubble, not a steady stream of bubbles, should occasionally appear on the surface. You will need to adjust the heat a few times to attain the optimal simmer.
Use a spoon to skim any additional scum that appears on the surface. After a while, rendered fat will form a sort of layer on top of the stock. If desired, you may spoon off any fat as it accumulates on the surface, although I wouldn't advise it. (It imparts flavor to the stock and will eventually be skimmed from the resulting stock anyway.)