Stage 1 - Prep and Thread the Skewers
Cut all your vegetables into the sizes specified rounds for the squash, zucchini, and corn; 1-inch pieces for the onion and peppers; stemmed (and halved if large) for the mushrooms. Consistent sizing matters here because it means everything cooks at roughly the same rate.
Thread the vegetables onto your 4 metal skewers, alternating types as you go for visual variety and even flavor distribution across each skewer. A good rotation might look like: squash, mushroom, red onion, bell pepper, zucchini, corn, repeat. Don't pack the pieces too tightly leave tiny gaps so heat can circulate around each piece rather than just the outer surface.
Drizzle the assembled skewers with olive oil, and season generously with sea salt and freshly ground black pepper. Turn them once to make sure both sides get coated.
Stage 2 - Grill the Skewers
Heat your grill to medium-high and spray the grates lightly with cooking spray before the skewers go on. Place the skewers directly on the grates and grill for 8 minutes per side 16 minutes total until the vegetables are tender and lightly charred at the edges.
Resist the urge to move them constantly. A few minutes of undisturbed contact with the hot grates is what produces real char marks rather than just steamed, pale vegetables. Use tongs to flip once at the halfway point rather than fussing with them repeatedly.
Remove from the grill once both sides show good color and the vegetables yield easily when pressed. Season to taste with an additional pinch of salt and pepper if needed, and serve with your chosen sauce or dressing on the side.