Stock is made in 3 phases: The first one is optional.
1) Roasting/ Blanching:
Roast the bones for extra flavour and colour. It also draws out impurities: discharge the fat and any muddy liquid after roasting the bones. Deglaze the roasting pan and add the liquid to the stock.
For extra purity the bones can be quickly blanched before making stock.
2) Drawing gelatine and nutrients out of the stock:
Cover the bones with cold water and bring the stock slowly to the boil. Vinegar is optional, I would not put too much. Restaurants don't use it. However, they use tomatoes, which are acidic, too. Acidity draws out nutrients from the bones but also heavy metals from a stainless steel pot.
Skim off the grey foam. Optionally repeat by adding cold water or ice to lower the temperature and bring to the boil again whilst skimming and removing more impurities which might make the stock cloudy.
Simmering lets impurities and fat rise to the surface where they can be skimmed off. Boiling will lead to impurities and fats stay in the stock. Furthermore, boiling your stock might draw out more nutrients but also draw out proteins too violently and make the stock go cloudy and bitter.
Always top up with water so the bones are covered. If they are not covered you are not drawing out nutrients/ gelatine.
Keeping the bones covered with water at all times ensures the maximum gelatine is drawn out of the bones. Although at the end of your 2, 4, 8, 16 or 20 hours you have drawn out all the gelatine that can be drawn out in that time, your stock is diluted (necessary to get all the gelatine out) and may require reducing.
If your finished stock does not gel
a) the bones have not been covered with water during cooking,
b) it has not simmered for long enough,
c) the bones are too chunky/ not cut up and nutrients and gelatine cannot be drawn out properly,
d) the bones are not gelatinous enough: as a general rule, try to include bones and tough cuts from tail, neck, head, joints, feet and wings, or
e) it has not been reduced enough.
If your stock is cloudy you have
a) used the roasted bones with all the muddy roasting juices,
b) used unsuitable vegetables,
c) not brought the stock slowly to an initial boil whilst skimming,
d) not skimmed the stock whilst cooking,
e) boiled the stock instead of slowly simmering,
f) not changed the pot when the sides of the pot got dried or burnt stains from reducing too much, or
g) smashed the boned before or during cooking and so released loads of impurities .
For a super clear stock clarify with egg whites, minced meat and mirepoix.