Remove heads from the dried sardines and split the bodies into two halves to remove the innards. Place the dried sardines into water and allow to soak overnight, or for at least 30 minutes if you need to save time, then heat over medium heat. When scum begins to form on the surface, simmer for about 5 minutes while skimming off the scum. Place a dampened paper towel over an all-purpose strainer and strain the dashi.