Preheat oven to 400ºF. Place tortillas on a baking sheet, and coat both sides lightly with cooking spray. Arrange in a single layer.
Bake for 4 to 5 minutes on each side or until crispy and golden brown. (They will continue to crisp as they cool.)
Meanwhile, cook turkey, onion and garlic in a large nonstick skillet over medium-high heat until turkey is browned, about 6 minutes. Stir in chili powder, cumin, salt, oregano, paprika, red pepper flakes, black pepper, and ¼ cup water. Simmer until water is evaporated.
Spread 2 tablespoons guacamole over each tostada shell; top evenly with turkey. Top each tostada with ¼ cup lettuce, 2 tablespoons cheese, and 2 tablespoons salsa. Serve immediately.