Summarize:

Drain the shucked oysters and reserve the liquid. Use a sharp kitchen knife to rough chop the oysters into smaller pieces.  Use a container of pre-shucked oysters for this recipe instead of fresh oysters. The actual oysters will be filtered out of the sauce later, so you do not need to worry about cutting them into precise sizes. Making them smaller can help release the flavor faster, though, which is why chopping the oysters beforehand is generally a good idea. Place the chopped oysters in a small saucepan with the reserved oyster juice and 1 Tbsp (15 ml) of water. Place the saucepan on the stove and heat it on high until the liquid reaches a steady, rolling boil. Stir the contents of the saucepan occasionally to prevent the oysters from sticking to the bottom of the pan. Reduce the heat to medium-low, allowing the liquid to drop to a gentle simmer. Cover and cook for 10 minutes. Keep an eye on the saucepan during this time. You do not need to stir the mixture inside, but you should make sure that the liquid remains at a steady simmer. Adjust the heat settings as necessary. Remove the saucepan from the heat. Sprinkle in 1/4 tsp (1.25 ml) of salt and stir well to combine. Pour the contents of the saucepan through a sieve or strainer. Save the liquid and discard the solid ingredients.  If you want to save the cooked oysters, you can do so by transferring them to a plastic or glass container with an airtight lid. Place the container in the refrigerator and store the oysters for up to four days. Pour the liquid portion of the mixture back into your saucepan after filtering out the solids. Add 2 to 4 Tbsp (30 to 60 ml) light soy sauce to the mixture and stir well. Pour 1/2 to 1 Tbsp (7.5 to 15 ml) dark soy sauce into the mixture and stir it in, as well.  Using both light and dark soy sauces will give the taste of the oyster sauce more depth, but if you only have one type of soy sauce on hand, simply use a full 2-1/2 to 5 Tbsp (37.5 to 75 ml) of that soy sauce, instead. If you aren't sure about how much soy sauce to add, start by adding a smaller portion. Taste test the results and add more soy sauce if you want a stronger flavor. Return the saucepan to the stove and heat it on high until the liquid reaches a rolling boil. Reduce the heat to medium-low and continue cooking for another 10 minutes. Keep the lid off the pan this time. The oyster sauce should thicken as some of the liquid simmers away, but the thickening process will be hindered if you keep the lid on. Let the oyster sauce cool for a few minutes before you use it. If you want to save the sauce for later, pour it into a plastic or glass container with an airtight lid and refrigerate it for up to one week.
Chop the oysters. Combine the oysters with reserved liquid and water. Bring to a boil. Simmer for 10 minutes. Add salt. Separate the liquid. Add the soy sauces. Boil and simmer for another 10 minutes. Use now or save for later.