Mix the buckwheat flour and bread flour together in a bowl. Add 1/2 of the water. Drizzle it around the perimeter so that the water doesn't reach the bottom of the bowl.
Mix rapidly until the dough is crumbly. If it gets stuck to the bottom of the bowl, scrape it off as you mix.
Add the rest of the water and continue mixing and scraping. Make sure the water is evenly distributed. The dough should look like cooked ground meat.
Press the dough together into a ball. Knead, always folding in the edges towards the center.
When the surface of the dough is no longer floury and feels smooth, it's done. Gather up the dough towards the center and form into a conical shape.
It should look like this, like an evenly formed cone.
Press down the dough from above the point of the cone. Roll the dough out with a rolling pin while dusting the surface of the pin, the dough and the work surface. When the dough is rolled out thinly enough, make into a rectangular shape, dust with more flour and fold in half. (The photo shows a dough using 250 g of flour folded in half.)
Fold over the dough from the top and bottom ends.
Make another fold from the side nearest you, so that there's a 1cm gap between the folded edge and the bottom edge of the dough.
The right tools help a lot. I bought the special soba-cutting knife and the board that helps me keep the slices even. Cut the dough into evenly spaced slices.
Put the noodles in boiling water. Mix in a figure-8 pattern with cooking chopsticks. When the noodles float to the surface, cook for an additional 30 seconds or so and they're done. Drain, rinse several times in cold running water, and cool in ice water.