In a bowl, whisk together the soy sauce, mirin, sugar, ginger, sesame oil, chili flakes, and garlic.
Pour half the marinade into a container or zip top bag. Add chicken and marinate for 1 hour or up to overnight(longer is better!).
Pour the remaining half of the marinade into a small saucepan. Bring to a boil and cook for 1 minute, stirring. It will thicken a bit. Set aside to brush on the chicken or use a a dipping sauce.
Soak the wooden skewers in water to prevent them from burning.
Slice scallions into 1 1/2-inch pieces(white and light green parts are best, closer to the stem. they keep their shape better when grilled).
Assemble the skewers, alternating scallions and chicken.