If using wooden skewers, soak them in water for 30 minutes to prevent them from burning on the grill. Fill a sheet pan with water, spread the skewers inside, and let them soak.
Cut the protein into even-sized 1-inch cubes. Chop the onion into quarters, then in half; cut the bell peppers into large 1-inch-square pieces, and the zucchini into ½-inch-thick rounds.
You can use a marinade for kabobs for the meat and veggies before assembling the skewers or season them quickly inside a large bowl or zip bag by mixing 3 tablespoons of your favorite seasoning and ⅓ cup olive oil, then adding the cut proteins and veggies to the bag, and tossing everything around to coat them with the seasoning.
Assemble the kabobs:
Thread the seasoned meat and veggies through the skewer, always poking through the middle of the ingredient to prevent it from sliding or falling off. With onions, I usually grab a couple of layers. Place the assembled kabobs on a baking sheet for easy transfer to the grill.
Grill:
Preheat the grill with the lid closed to medium-high heat. If your grill has a temperature gauge, it should read between 375F to 400F, otherwise, preheat for 10 to 15 minutes. Use grilling tongs to grease the grates by wiping a paper towel soaked in oil over the grates. If you used the oil + seasoning mixture, this isn't as crucial since your ingredients will be nicely coated. This step is especially important when using a marinade.
Place the kabobs on the grill across the grates (not parallel). Sear the first side with the lid open for a few minutes, then close the lid and cook for about 7 minutes. Open the grill, flip the kabobs over by picking them up from the middle using grilling tongs. Then, close the lid and cook the second side for 5 to 7 more minutes. Total cooking time will vary depending on the type of protein and thickness. Chicken breasts will need to be cooked to 165F and beef between 140-145F.
Remove the kabobs from the grill to a tray, then cover them with foil to keep warm while you finish grilling.