Wipe kombu with a damp paper towel to remove any dust. Do not wash it. Use a sharp knife to make a few cuts in the strip of kombu. Place in a bowl with the water. Let sit 4-12 hours at room temperature (the longer you soak kombu, the more flavorful the broth), or overnight in the refrigerator. Remove kombu.
Alternatively, place water and kombu in a medium size pan over medium heat. Remove kombu just before water reaches a full boil. Turn off heat.
Dashi can be frozen (I do this so I can make miso soup and other soups whenever the whim strikes me).