Sit square to the camera, about an arm away. Push your hair back off your face and neck. Bare face if you can — foundation is someone else's guess at your undertone, and we would only be measuring theirs.
Dim the room. In a moment the screen will go white, then black, twice. That is not a flourish. Your camera is white-balancing on your face right now, which is the exact thing we are trying to measure, so we cancel the room by using the screen as a known light and subtracting.