Perhaps the most important item required for custom content creation is Belgabor's Sketchup plugins. The original download is not available any longer, so I have them uploaded here. Credit for these plugins goes to Belgabor of the RCT3 community. Click the image to download.
This tutorial covers some basic concepts of modeling and how to export models from Sketchup. Click here to download.
Tutorial two covers the basics of importing models into RCT3. I recommend reading tutorial one before the second to be sure you have learned the proper way of exporting and to avoid confusion and frustration down the line. Click here to download.