This is set in the print dialogue window. Go to File > Print.
On Windows, check the Back to front box:
On Mac, (if necessary) expand the Print window by clicking Show Details.
Choose Final Draft from the dropdown menu in the middle of the window. Check the Back to front box:
NOTE: If you have your printer driver set to print backwards as well, the script will print out front to back. Make sure only Final Draft or your printer driver is set to print backwards -- not both.