If you download a .zip file, it usually contains a folder with 2 sub-folders: scenery and texture. Copy the folder that contains the scenery and texture folder to "\Flight Simulator 9\Addon Scenery\". Some add-on scenery packages may only contain a scenery folder. If that's the case, don't create an empty texture folder yourself, because it will cost frame rate and you don't get anything with it. Now start FS, go to settings > scenery library and click on "add area". Browse to the Addon Scenery folder in your FS main folder, select the folder you just added and click "OK" and then "OK" again. Exit FS, you will be able to see the new scenery when you restart FS.

Scenery developers usually supply you with a readme file within the package. Be sure to read it, because the installation may be different and/or more complex.

Scenery for FS2002 may be compatible with FS2004, but not always.

If you have 2 different scenery packages of the same area or airport, landclass and airport scenery for example, it's best to give the airport scenery higher priority. The priority works like layers, the highest priority scenery is placed on top of the lower priority scenery. It is also possible to switch between 2 sceneries of the same airport if you enable the one you want to use and disable the other one, or visa versa.

If you don't like the addon scenery anymore, delete it from the scenery library menu, exit FS and delete the folder in your Addon Scenery that contains the scenery you just deleted from the scenery library menu. FS will load the default scenery next time you start FS.

AFCAD files

AFCAD files are .bgl files that start with "AF2", they contain airport data like runways, taxiways, etc. Author Lee Swordy created a small program called "AFCAD" that let you edit those files. That's why those files are called "AFCAD files", you can edit an airport like a CAD program.

Downloading new AFCAD files becomes handy when you need more parking space at an airport, or the layout of the taxiways is in inaccurate. Addon AFCAD files should be copied to "\Flight Simulator 9\Addon Scenery\scenery\" . For FS2004, we use AFCAD2 files. For example, a new AFCAD file for the Seattle Tacoma Airport could be named AF2_KSEA.bgl. When you've copied the new AFCAD file to the scenery folder in the Addon Scenery folder, the next time you start FS, the new airport data is loaded. Just remove the file if you want the default airport layout back.