Mix the liquid ingredients and let them simmer while removing the scum, till the mixture boils down to 200cc, reducing by about 80%. Add soy sauce (200g) and raw sugar. Many restaurants use raw sugar, since it tends to make less scum than white sugar. Mizuame (millet jelly) is also used by some restaurants instead of raw sugar. After you add sugar, let the sauce simmer over low heat and be careful not to burn it, till about the 5% of the liquid evaporates.
Once the sauce is caramelized, it is done. Usually, the sauce is stored in the fridge and brought to room temperature when it is used.