1. Select File>Save As… (if you get a warning about the file being referenced by another document, just say OK.
2. Change the Save as Type to STL (*.stl).
3. Click the Options… button, which will
open the Export Options dialog box, most of which is shown to the right.
Depending upon the final intent of the part, you will likely want to either select Fine in the Resolution section, or Custom, which will allow you to smooth the STL even further. The Deviation and Angle sliders correspond to the maximum distance and angle errors that will be allowed between points of the triangular mesh and the original model. File size will increase dramatically as you increase the resolution.
To see a preview of your triangulated mesh, click the checkbox next to Preview.
Click OK once you’re happy with your mesh.
4. Click Save to save your STL file. You may have to click Yes to then actually save the file.