Answer: With your eyes!
All that is required is a pair of eye-tracking glasses (in this case Tobii Pro Glasses 2), a small computational unit, and a drone. The glasses contain an inertial measurement unit (IMU), which a smart deep neural network uses the data from to determine how far away the drone is and where the wearer is looking.
The system was developed by engineers at New York University, the University of Pennsylvania, and the U.S. Army Research Laboratory. The team envisions (pun intended) their technology being useful for novice drone pilots, allowing them to fly in situations that might otherwise call for a more experienced pilot.