In terms of scenery design (Custom aerial textures etc) FSX is a lot easier to get 'into' than FS9...the SDK in FSX is a lot better than Fs9. Almost all of the tools youll need for FSX development are included in the SDK (Gmax, resample.exe, bglcomp.exe, tmfviewer.exe, Annotator.exe etc)
Step one is read the SDK documents....depending on prior knowledge there is probably a lot of new concepts to grasp. Second, set up Gmax....this can be a bit of a mission in itself. 3rd- Make a box in GMAX, texture it and try to get it to show in FS.....this can be a bit of a mission in itself. 4th- Make a more complex object in GMax and texture it.....this can be a bit of a mission in itself etc etc
Good luck