If you wish to make soba from scratch, you must first invent the universe. I didn't go quite that far, but I did make soba noodles with my host parents from powder and water on Saturday. We had a big shiny bowl and mixed up the powder/flour/whatever by hand and added water until it got doughy.
Then split it into two chunks,
kneaded it a bunch,
flattened it out,
folded it, and cut it into noodles that ended up being shorter and more brittle than they were supposed to be.
Then it was just a matter of cooking them up, throwing them in cold water, and then eating them by dipping them in a cup of some sort of flavorful liquid. They were definitely shorter than they were supposed to be, but they still tasted good. And that's about all there is to it!