Follow these simple steps to prepare and cook Japanese Chicken Yakitori:

1. Prepare the Marinade: In a bowl, combine soy sauce, mirin, sake, brown sugar, and sesame oil. Mix until the sugar is dissolved.
2. Cut the Chicken: Cut the chicken thighs into bite-sized pieces, about 1-inch cubes.
3. Marinate the Chicken: Place the chicken pieces in the marinade and let them sit for at least 15 minutes. This allows the flavors to penetrate the meat.
4. Prepare the Skewers: If using vegetables, cut them into similar-sized pieces. Thread the marinated chicken and vegetables onto the soaked bamboo skewers, alternating as desired.
5. Preheat the Grill: Preheat your grill or grill pan over medium-high heat.
6. Grill the Skewers: Place the skewers on the grill and cook for about 3-4 minutes on each side, or until the chicken is cooked through and has nice grill marks.
7. Baste with Marinade: During the last few minutes of cooking, brush the skewers with leftover marinade for extra flavor.
8. Season: Once cooked, sprinkle with salt and pepper to taste.
9. Garnish: Top with chopped green onions for a fresh finish.
10. Serve: Remove the skewers from the grill and let them rest for a couple of minutes before serving.

These steps will guide you in creating this incredible dish effortlessly.