Hello!
Could you tell us which game you are playing? The process is a bit different for each game, but the general idea is to get the files you've downloaded from your Downloads folder into the proper game folder.
For
OpenRCT2, for example, the folder can be located in
Documents -->
OpenRCT2 -->
track (if it's a ride),
save (if it's a park), or
scenario (if it's... well, you get the idea).
For standard
RCT2, the folder is a bit harder to find. For example, you may find it in
Local Disk -->
Infogrames Interactive -->
RollerCoasterTycoon 2 --> Tracks /
Saved Games /
Scenarios (depending on the type of file). The step marked by the green arrow might include having to click a button that says
Compatibility Files in order to see custom tracks, but while this step is probably necessary for uploading your own content, it may not be if you're just downloading.
For
RCT3, refer to
this guide for info on all of the various folders.
Copy the file from your downloads folder into any folder that is marked in yellow in the text above.