IT's in the SDK

However, most everyone starts with copying over the files and hierarchy from an existing (usually stock) plane with as close characteristics as the model you're creating, and modifying it from there.

In other words, say you're creating a brand new Cessna 162.  You have your model and textures, but you want something to be able to see and fly in FS.

You'll create your cessna 162 folder and sub-folders and copy the aircraft.cfg,  *.air,  model.cfg, panel.cfg, sound.cfg from the Cessna 172 and start changing the different parameters (especially in the aircraft.cfg file) to match YOUR model/design.

Obviously you have to change aircraft names, etc.

The SDK is your friend, read it, learn it, become as one with it....