In the File tab, click on Print.
In upper left corner of the Print Preview dialog make sure "Print on One Page" is selected. Choose the size and orientation of the page that best suits your diagram. SmartDraw will automatically scale your diagram to fit on a single printed page.
With a larger diagram, the scaling needed to reduce your diagram to a single page may make your text too small to read. Here are some tips on how to deal with larger diagrams.