How can you navigate "Writing" onto FlightRadar Plans?
Most Robin 400s don't have autopilots, but some do. Whether it has an autopilot or not it was almost certainly navigated using a pre-set course which was fed into a GPS. One would use flight planning software to create a course using a series of waypoints which would then be uploaded to a built-in or handheld GPS, and then either flown by hand or followed by an autopilot. 

You can see that the curves are mostly a series of small straight lines, I think that these straight lines are the space between waypoints, it could be the refresh between the FlightRadar inputs causing the lines though.