This dashi is for me the backbone of numerous Japanese dishes and I have been perfecting mine over the course of many years. For best results soak the konbu for 24 hours. If you are not able to do that, you can use the quick method listed below.
Combine the water and the konbu in stainless steel pot, let sit for 24 hours. Remove konbu and slowly heat until almost at simmer, turn off heat and add the katsuobushi. Let sit for 20-30 seconds and strain.
If you are not able to soak the konbu for 24 hours, soak for at least 3 hours, then slowly bring to almost a simmer. Remove konbu, add katsuobushi and turn off heat. Let sit for 20-30 seconds and strain.