Instructions
Download a custom tracked ride from any of the online sites that host them, such as RCTDatabase. Most of the rides are in the roller coaster category.
Unzip this file into the RCT3 rides directory, under "Rollercoaster Tycoon 3 Platinum\Rides\TrackedRides." If the creator of the CTR has special instructions for installation, read the "Readme" file in the ride's .zip file.
Open "Rollercoaster Tycoon 3," then select a new game, scenario or sandbox park. Open the "Rides" tab, then "Rollercoasters," then scroll down to find your CTR. It may have its own logo, or a plain logo that says "CTR."