Pick through your beans and remove any sad-looking beans, either shriveled and/or blackened.
Rinse the beans thoroughly and then cover them with 2” (5 cm) of cold tap water above the soybeans. Soak them overnight, up to 24 hours. If your room is very warm in the summer, soak the beans in the fridge.
Once the beans are fully hydrated, drain the water and rinse the beans once more.
(Optional) You can remove the skins of the beans by rubbing them between your fingers. Fill the bowl with water again, the skins should mostly float above the beans and you can pour them out. This will take 3 or 4 rounds. Some people prefer the beans without their skins because they feel they make the soy milk taste beany or bitter. I tried both and I don’t think there is much of a flavor difference so I don’t think this step is necessary.
Before begin cooking, set up a strainer lined with layered cheesecloth, muslin, or a coffee filter (if using a coffee filter you may have to change it out once or twice during the straining process) and place it over a heat-proof vessel.
Add the soybeans into a blender with 4 cups of water. Blend on high until smooth. Depending on the size of your blender, you might need to blend in two batches. In this case, make sure you use 2 cups of water to blend each half of the beans so you will use 4 cups in total.
Pour the mixture into the tallest pot available. Add 3 more cups of water to the pot and set to medium-high heat.
Stir constantly with a wooden spatula, scraping the bottom as you do to prevent burning. Once the soy milk starts to boil, turn to medium-low heat and let boil for 20 minutes. As you cook the soy milk, the foam will rise. Keep a close eye on the pot. If it looks like it’s about to boil over, turn the heat to very low or off, and stir until the foam dies back down. Once the foam has receded, turn the heat back on and continue cooking, stirring constantly, until the 20 minutes are up.
Turn the heat off and stir for another minute to ensure the bottom doesn’t burn, then pour the soy milk over the lined strainer to filter out the okara. Make sure to stir and press the soy pulp with a spatula to ensure all the soy milk gets through. When most of the liquid has left the soy pulp in your strainer, scoop it into a separate bowl and pour more of the cooked soy milk into the strainer. Once all the liquid is strained off, put all the collected pulp back into the strainer and squeeze as much soy milk from it as possible (if using coffee filters be careful not to tear the filter).
Add a pinch of salt and/or sugar to taste. These ingredients will just balance and enhance the flavor of the soybeans. You can sweeten the soy milk by adding more sugar, if needed.