Instructions
Select the scenery from a website with free downloads such as FreewareScenery or Simtegral. These sites have descriptions of the downloads, so you will know exactly what you are getting before you start your download.
Click the download link for the scenery, and select "Save File" from the menu that appears. The archived file will automatically start to download to the "Downloads" folder located in "My Documents.
Right-click on the archived file you downloaded, and select "Extract All" from the menu. When asked to select a destination, click the "Browse" button and select the Windows Desktop. You will now have the files in a temporary location where you can easily copy it from.
Navigate to the folder on your hard drive where "FSX" is installed and open the "Addon Scenery" folder inside it.
Copy the content from the folder on your desktop to the "Addon Scenery" into a subfolder with a descriptive name. If the download only contains a "Scenery" and "Texture" folder, create your own descriptive folder then add these two folders to it before placing it in the "Addon Scenery" folder. The goal is to have all the scenery files together in a descriptive folder to make it easier to select and use from within "FSX."
Start your copy of "FSX," and select "Settings" from the menu. Click "Scenery Library" then "Add Area." Double click the "Addon Scenery" folder and select the folder containing the scenery you have downloaded. Click "Ok" and the scenery will be installed.