Spread out your baking soda onto a clean baking sheet.
Bake the baking soda in the oven for an hour.
Leave to cool and store in a clean glass jar with a tight-fitting lid.
Ramen noodles
Add water, the kansui powder, and salt to a measuring jug, then stir to dissolve the powder and salt.
Weigh out the flour and add to a medium-sized bowl, then pour in the water mixture.
Mix till it forms a dough. You can use a little bit of extra water if you need to. (be careful though, you do not want to add too much water, since you want a very stiff dough that is quite dry)
Knead and work the dough for about 10-15 minutes or till it is smooth and the surface feels almost rubbery. (I know it is hard work, but I promise the noodles are worth it)
Cover the bowl with either a damp tea towel or put the dough in an airtight lunch box. This to prevent the dough from drying out while it rests.
Let it rest for an hour (although I have rushed it sometimes, and it still works alright, just not as well)
When rolling out your dough, use plenty of cornflour to prevent it from sticking to itself.
Pasta machine: Roll out the dough till it is thin enough to go into the pasta machine. Slowly work your way up in thinness with your pasta machine. I prefer them at thickness 4.Feed the sheet of dough through the cutting rollers, and either hang them up or create bundles of noodles, until you are ready to use them.
Elbow grease: Roll out the dough till it is thin enough to see your hand's outline when you hold your hand behind the dough.With plenty of cornflour to prevent sticking, fold the sheet of dough for easy cutting.With a sharp knife, ideally with a flat cutting edge, cut your dough into strips.Save them in little bundles until you are ready to use them.
Cook the noodles in plenty of boiling water in a big pan (to prevent the pan from boiling over)
Cook them for half a minute, to a minute, so that they will keep their distinctive "bite".
Then serve them in a steaming bowl of bone broth with your preferred toppings.
Notes
If you want to save the noodles for a later day, you can keep them in an airtight container for 2 or 3 days in the fridge or a couple of months in the freezer. Although I find that when I freeze my noodles, they lose some of their bite, they're still tasty though.Baking soda and baking powder are different! Make sure you use sodium bicarbonate.