Put the baking soda and salt in a small bowl, add boiling water and dissolve (This de-gasses the baking soda.) Add cold water and cool down until it's a bit cooler than body temperature.
Put the all purpose flour in a large bowl, make a well in the center and add the water with salt and soda in it from Step 1. Mix with your hands until crumbly.
It will be very dry and hard to mix to start, but keep kneading patiently and try not to add more water.
When the dough comes together, put it in a large plastic or Ziplock bag, seal and step on it with your feet to develop the dough. (When the dough becomes flat, take it out, fold it into quarters and step on it again.)
Repeat the stepping and folding process 3 times, and leave the dough to rest for 2 to 5 hours.
If you have a pasta machine, set the dial to 4 or 5, and cut the dough into thin noodles. If you don't have a pasta machine, roll the dough out with a rolling pin to about 2 mm thick, and cut into 2mm noodles.
Dust the noodles evenly with cornstarch (or katakuriko) to prevent the noodles from sticking to each other.
If you divide the noodles into 150 to 160g portions, you'll have 5 servings exactly. If you prefer ripply noodles, squeeze the noodles well.
Boil the noodles in plenty of boiling water, taking care not to let it boil over, for 1 and a half minutes.
To use the noodles as tsukemen (dipped ramen), rinse well in cold water to remove surface stickiness. If you're serving the noodles hot in soup, eat right away to prevent them from becoming soggy.