What's a fast way to make Furikake from Katsobushi used in making dashi
There's no set recipe for furikake, but you will commonly find sesame seeds (both black or white), katsuobushi (bonito flakes), and seaweed.
For dashi leftovers, just cut the kombu/katsuobushi into small pieces and toast in a dry pan with some soy sauce/sugar/salt/MSG to taste until the mixture is dry. You can toast the sesame seeds separately or together with the dashi leftovers, then add nori (cut or crumbled into small pieces). Let cool, preferably on a tray to help it dry out more.
For quantities, here is a recipe for reference.