In a large mixing bowl, whisk together olive oil, lemon juice, minced garlic, oregano, cumin, paprika, salt, black pepper, and red pepper flakes until well combined. Pro tip: Let the marinade sit for 5 minutes to allow the flavors to meld together.

Add the chicken pieces to the marinade, tossing to coat evenly. Cover and refrigerate for at least 30 minutes, or up to 4 hours for maximum flavor. Pro tip: Don't marinate longer than 4 hours as the acid can start to break down the meat texture.

While the chicken marinates, cut the bell peppers and red onion into 1-inch pieces. If using cherry tomatoes or zucchini, prepare those as well. Pro tip: Keep vegetable pieces similar in size to ensure even cooking.

Soak wooden skewers in water for at least 30 minutes to prevent burning on the grill.

Remove chicken from marinade, reserving any excess marinade for basting. Thread chicken and vegetables alternately onto skewers, leaving small gaps between pieces for even cooking. Pro tip: Don't pack the ingredients too tightly – air circulation helps everything cook evenly.

Arrange completed skewers on a large plate or baking sheet, ready for grilling.

Preheat your grill to medium-high heat (about 400°F). Clean and oil the grill grates to prevent sticking. Pro tip: Use tongs and a paper towel dipped in oil to safely grease the grates.

Place skewers on the preheated grill and cook for 12-15 minutes total, turning every 3-4 minutes to ensure even cooking on all sides. Pro tip: Baste with reserved marinade during the first half of cooking for extra flavor.

Check for doneness using an instant-read thermometer – chicken should reach an internal temperature of 165°F. Pro tip: If some pieces are cooking faster than others, move them to a cooler part of the grill.

Remove from grill and let rest for 2-3 minutes before serving. Pro tip: This resting time allows the juices to redistribute throughout the meat.