Japanese soup broths are generally similar in flavour. All use ichiban dashi, and the final result is called kakejiru.
For the Ichiban dashi:
2 quarts water
5 six inch squares of kombu (kelp)
1 cup tightly packed katsuobushi (bonito fish flakes)
Wipe the kombu squares. Place in a pot of the water.
Bring it to a near boil and right before it boils, remove the kombu. Add the katsuobushi and boil. When it comes to a boil, turn off the heat and skim foam.
Strain the stock and it’s ready to use.
For flavouring the stock to make the broth:
1 quart ichiban dashi
1 1/2 tbsp sugar
1 1/2 tsp salt
2 tbsp soy sauce
Slowly boil all the ingredients over medium. Serve with the soba or udon. Remember that soba (buckwheat noodles) should be boiled a little longer than udon, while somen is quick to cook.
In a large bowl, pour the soup and place the noodles into it. Top with finely sliced green onion and any other toppings you prefer.