SLTr is a library for Processing that write PShape object from Processing's core library to STL files for 3D printing.
To add this library in Processing got to Sketch > Import Library > Add Library and search for STLr. If it doesn't show up, look up how to install with just the .jar file and get the STLr.jar from src/stlr/library.
This library comes with an example of how to use it in examples/Splionize. If you have Processing just download the folder and open the .pde file in the Processing Development Environment.