Rhino

Once the part or assembly is created in 3D and is in positive space:
  1. File > Save As
  2. Select File Type > STL
  3. Enter a name for the STL file
  4. Save
  5. Select Binary STL Files

For additional questions refer to your manual, MIS department or the Maker of the software: McNeel

Select your CAD program in the pulldown menu to view the recommended settings for exporting an STL file