Don’t worry, I am holding your hand.
PX4 Research Log  – PX4 Off-board Control with MAVROS (1)
Companion computer is a very efficient way to control your PX4 vehicle in off-board mode. We have tested various methods and we have concluded Robot Operating System (ROS) is the best way. Here is a series of tutorials guiding you to achieve off-board control. What you need: A Pixhawk or a Pixracer A Raspberry Pi