Creating Thai Grilled Chicken is simple if you follow these steps:

1. Prepare the Marinade: In a medium bowl, combine fish sauce, soy sauce, brown sugar, vegetable oil, garlic, ginger, lemongrass, and red chili flakes.
2. Marinate the Chicken: Place the chicken thighs in a large zip-top bag or a shallow dish. Pour the marinade over the chicken, ensuring each piece is well coated. Seal the bag or cover the dish and refrigerate for at least 30 minutes or up to overnight for maximum flavor.
3. Preheat the Grill: Heat your grill to medium-high heat. If using a charcoal grill, ensure the coals are hot and glowing.
4. Grill the Chicken: Remove the chicken from the marinade, allowing excess marinade to drip off. Place the chicken on the grill and cook for about 6-8 minutes per side, or until the internal temperature reaches 165°F (75°C).
5. Rest the Chicken: Once cooked, remove the chicken from the grill and let it rest for a few minutes to allow the juices to redistribute.
6. Garnish and Serve: Slice the chicken and garnish with fresh cilantro. Serve with lime wedges on the side for added flavor.

Following these steps will ensure that your Thai Grilled Chicken turns out incredibly flavorful and juicy every time.