How would one go about creating my own aircraft for FS9 or editing the ones that are currently there?
You can build it with either of these.
Or free on Disk #1 GMAX, a little more difficult to use (I understand) but the aircraft are better looking and seem to be accepted better by the sim (my opinion).
I think you have to first decide which avenue you want to go down. They are two related yet completely different universes - paradox, I know.
Creating your own aircraft requires a lot of work, there is a lot more to it than just creating the shape if it's to be realistic.
Editing existing aircraft can range from very easy to hard, depending on what you are trying to edit.