Fairings are most effective on Thule Aero Feet applications, where the front bar is mounted close to the top edge of the windshield, usually around 7" back. They will do a great job of reducing wind noise in these applications.
Mount the fairing first, before mounting any of your accessories because the fairing has specific mounting points. You will want your fairing to be centered on the car, and this can be done with a good eyeball from the front of the car.
Be careful, though, rack applications that are using multiple accessories on the roof may be interfered with when using a fairing.
