Miso marinade: As suggested in the recipe, I used sweet "saikyou" miso 西京味噌 (1/4 cup) but if you use regular white or red miso, you could add sugar. I added mirin (2-3 tbs) to the miso until a thick sauce consistency was reached. After I smeared the miso marinade on both sides of the fillet, I wrapped it with plastic wrap and put it back in the Ziploc bag. I marinaded it for 6 hours before grilling. The original recipe calls for 3 days of marination.
Grilling: I used a Weber grill with lump charcoal. As usual, I started the fire using a Charcoal starter chimney. I spread hot coals over only half of the grill to make hot and cool areas. After cleaning the grill, I sprayed Weber-brand grilling spray (this does not flare up even with hot coals underneath). I started the meat side down on the hot area of the grill for 2 minutes or so and flipped it over to grill the skin side for one to two more minutes. Because of the sugar content of the miso marinade, it is very easy to char the fish like I did here (image below right, but it still tasted good). I moved the fillet to the cool side of the grill and put the lid on and finished cooking for 2-3 more minutes.