You are going to want to get a large stock pot and place this on the stove.
Put the carcass, 1 tbsp of ginger, 6 cloves of garlic, 1 onion cut into quarters, ½ tsp of peppercorns, and 2 tbsp of oyster sauce in the large stock pot and cover with water.
Bring to a boil, then lower the heat and simmer for a minimum of 3 hours (do not cover).
While the stock is cooking, skim the top periodically for scum.
After 3 hours, if you choose to cook for a longer period, know the stock will become more concentrated the more it cooks.