Instructions
Go to a Rollercoaster Tycoon 3 custom scenery download site. Choose the file of your liking.
Download the custom scenery. Allow the download to complete. Unzip your download.
Copy and paste the contents of the download into the Rollercoaster Tycoon 3 "Style/Themed" folder. You can now use your new custom scenery inside of Rollercoaster Tycoon 3.