It's generally situated on top of or near the intake manifold, toward the rear of the engine. It will be a small, usually electrically operated, valve or actuator with wiring connected to it. A vacuum line may also be connected depending on the specific design of the system.
Because its exact placement can vary slightly, using a repair manual specific to your vehicle's year and engine is highly recommended. A picture of the valve in a repair manual will greatly assist you in locating it.