There are 3 levels of customization that you can do in X-Plane.

The first is changing the artwork for the airplane to customize the instruments, instrument panels, paint, sounds, and anything else about the airplane you can imagine. This is all VERY easy to do... all you need to do is customize the requisite graphics files with a copy of Photoshop (or any other graphics editor of your choice) and modify or create any sound file you like with the sound-editor of your choice, save it with the correct name and in the right place in your aircraft folder, and voila! Your aircraft will be customized with the new sound and graphics that you create.

So, what filenames and folders should yo use to tweak the airplanes? Easy! See the "EXAMPLE PLANE" that comes with every copy of X-Plane 8.00... it has some custom graphics to give you the easiest possible example to quickly get started on your own designs!

The next level of customization is to actually change certain variables inside of X-Plane or get data out of X-Plane. This is typically used to drive motion platforms, drive external graphics displays, or drive X-Plane with your own flight model. For details on how to do this, see the "UDP Reference.html" document in the "INSTRUCTIONS" folder that comes with every copy of X-Plane 8.

The final level of customization for X-Plane is PLUGINS. This is actually a software kit that lets you read and write any variable in the entire simulator. Learn about it here: http://www.xsquawkbox.net/xpsdk/