We make odashi (soup base) quite often because we eat Japanese food quite often.  This soup base can be used for miso soup, udon noodle (if making hot udon with soup, but you can use this as base for cold dip sauce), base for braised food, anything Japanese really...  What you make soup base with differs significantly from region to region.  Where I am from, west region of Japan, we use significant amount of dried konbu (sea kelp) and bonito flakes.  But other region uses tiny dried fish, dried shiitake mushroom in addition to kelp and bonito flakes and for strict vegetarian, skip bonito flakes and put dried soy beans.  

Traditionally, we use ton of bonito flakes, but I usually use ready made packets because of convenience.  But once you use real bonito flakes, especially freshly shaved ones, you can tell it tastes much much much better!  I feel funny to put recipe on how to make odashi since it feels sooo basic, but my American BF felt it is helpful, so here it is;

Ingredients

• Dried konbu (giant sea kelp).  Make sure to use konbu specifically for soup base making.  You cannot replace with seaweed.
• Bonito flakes (in this case I cheated and used ready-made packets)
• Water.  If you have a choice, best to make with soft water, not hard water

Directions (these are very loose directions I rarely measure anything these days)

1. At least 3 hours before hand, add dried konbu and ready made dashi packets to a pot of water. I usually do this first thing in the morning, if I know I am going to make something for dinner that requires it. So it can sit up to 9 hours. The longer it sits the more potent the umami :)
2. When you are ready to use it, heat the odashi just until it steams and bubbles start to form - DO NOT boil!
3. Strain or remove konbu and bonito flake [packs] before using.  If you are frugal like me, you can also make braised konbu to eat with rice later!
4. If using bonito flakes (not dashi packs), put them in after removing konbu and also don't boil so that soup base remains clear.  Then strain bonito flakes, make sure not to squeeze dashi out otherwise, soup has some bitter taste, just strain it with kitchen paper on strainer.