Before diving into technical details of the whole developing process, a taste of flying is always interesting.
You can buy a PhenixPro Devkit flight controller first;
Build a testbed quadcopter UAV, wiring all components with flight controller;
Download the source code and build the firmware;
Program the flight controller by coping executive binary in the booting TF card;
Configure all parameters of the quadcopter with ground station, like Mission Planner