You can also use a pasta machine, nihachi soba I do encourage you to try making the dough by hand. I recommend using a digital scale to measure the flour.
Dampen a paper towel or dish cloth and place underneath the large bowl to stop it from moving. Sift the buckwheat and all-purpose flours into the bowl. Set aside 2 tablespoons of the filtered water and pour the rest in the bowl. Using your fingertips, work the water into the flour until well combined. Scrape any excess dough off your fingers, then continue to work the dough until it forms a crumbly mass.
Using the palms of your hands, knead the dough in a counterclockwise direction, then back to clockwise repeatedly: If the dough feels dry, add a tablespoon of water and continue kneading until the crumbly mass forms dough balls the size of a pebble. Gather the small balls until they form balls the size of walnuts. Eventually, the large balls will combine to form one piece of dough. It should be smooth with no visible cracks.