Measure the nigari into a dry cup. Add the hot water and stir until the nigari has dissolved. Set aside.
Pour the soy milk into a large pot and bring it to a boil over high heat, stirring occasionally (so that the milk doesn’t burn).
When it begins to boil, reduce the heat and let it simmer for 3 minutes.
Remove the pot from heat. Add half of the nigari mixture to the soy milk, stirring with a wide wooden spatula in a gentle whirlpool pattern, 5 or 6 times. Then bring spoon to a halt upright in the soy milk and wait until all movement ceases. Then add the remaining nigari mixture, and this time stir a gentle figure eight pattern. When you notice that the soy milk is beginning to coagulate, cover the pot and let it sit for 15 minutes.
Line two small tofu molds (or small colanders) with clean tightly woven cotton cloths and set the molds in a shallow baking dish, or in the kitchen sink.
Using either a skimmer or a slotted spoon, scoop out the curds (the curds are the solid bits and the liquid is the whey) and transfer them into the cloth-lined tofu molds.
Fold the cloth over the top of the curds, set the top (or a small plate, if using a colander) on the curds, and place a weight of about 1 1/2 pounds on top.
Let the tofu sit 15 minutes. Fill a large bowl with cold water. Gently transfer the tofu, still wrapped in the cloth, into the bowl of water, and unwrap it. Carefully remove the cloth and let the tofu sit in the cold water for 30 minutes.
Serve the tofu immediately or store it in fresh cold water in the refrigerator for up to several days.