Soak the soy beans in water overnight (8 to 12 hours). Be sure to use enough water so the beans can double in size.
Prepare the molds using one milk carton per mold (you can use cheese molds if you have them). Cut off one side of the carton, and punch numerous holes in the sides and bottom to allow liquid to escape. Reinforce the molds by using a pair of longs sticks (one on each side) holding them together with rubber bands on the ends.
You should have one piece of cheesecloth per mold which should be big enough to fit squarely into the mold and cover the tofu inside.
The next day, pour out the water, and pulverize the beans in a blender using enough water to cover the beans (you will probably need to do this in four to five batches). Place a piece of cheesecloth in a colander, and pour the liquid through it into a large bowel. You can do this process much faster with a juicer by the way. Here is one demonstration how it is done. You can also boil the bean first then blend it.
The liquid that passes through is called soy milk and will become the tofu. The solid material that remains in the cheesecloth is called okara and is useful for various Oriental dishes.
Bring the soy milk to a low boil and lower the temperature, skimming off any foam that collects on the top. Boil for 20-30 min. stirring (and skimming) constantly. While it is boiling, dissolve the bittern (the leftover "stuff" that is formed when you extract salt from seawater) in a small quantity of water.
For each of the molds, fit the cheesecloth squarely into the corners. When the temperature of the soy milk cools to 70C (158F) pour in the bittern and stir gently. When the tofu starts to separate from the water (almost immediately), pour the mixture into the molds and cover with cheesecloth.
Next, place the plastic wrap covered newspapers on top of the tofu (it should fit squarely) and place weights on top to press out the liquid. After about 30 min., remove the tofu from the molds. This should be done under water in a large pot or tub. Store the tofu submerged in water.
Bittern is the bitter liquid remaining after common salt has been crystallized out of sea water and is source of magnesium, bromine, iodine, and other compounds. To make tofu, the edible calcium sulfate should be used, instead of the industrial one.
The edible calcium sulfate is used as a kind of Chinese medicine and is available in Chinese drug stores, Chinese shops, other online stores, or Japanese and Korean stores may carry them as well.
Calcium sulfate is also called Fibrosum or Gypsum in academic field. Anhydrous calcium sulfate contains CaSO4, while hydrous calcium sulfate contains CaSO4.2H2O and is semi-transparent. If only hydrous calcium sulfate is available, you may fry it in a pan or heat it in a microwave to get rid of the water to make it anhydrous.
Then quickly pour the soymilk into the bucket that contains 250ml lemon juice. Very important: Do not move or stir the mixture at all. Let the mixture set for 5 minutes to coagulate into soya pudding, then continue to follow next step in the directions.