Virtual - Testing & HITL simulation

Hello good people!

I have always dreamed of making my own plane models and testing them virtually like xplane, flight gear or any open source medium like cesium. But I’m really clueless about where to start. I have so many questions like how to model the plane, how to apply stability parameters and how to connect with mission planner.

If anyone has prior experience on that, please share your wisdom with me so that I can create and test my plane virtually.

I appreciate your help in advance.