The Lazy Eight maneuver is a flight trajectory that requires symmetrical climbing and descending turns through 8 reference points. So, it can be viewed as a tracking problem. Feed in the Lazy Eight trajectory to the existing flight control system, and the aircraft should follow the reference trajectory.
If you have a model of the trajectory, compare the vehicle state with the trajectory to generate an error signal that can be feed into a PID controller that has access to the vehicle effectors.
While I've never modeled a Lazy Eight maneuver, I have had some success flight testing typical maneuvers on a supersonic half-scale research UAV.