Personalize text and photos. Once finished click on 
button on the top right corner of the Card Creator page. From the pop-up “Print” menu, select printing properties:
Card Size: Full Size or Half Size. We recommend Full size as it fits 7 x 5 envelope.
Paper Type: Letter Size paper (used in the US & Canada), or A4 (used in the rest of the world.)
Printer Type: Single Side Printing or Two Sided Printing
Two Sided (duplex) Printer
As most new printers can be set to automatically print on both sides of the paper, selecting this option will save you printing one side of the card at a time. To print a card on a 2-sided printer, make sure you enable that feature in your printer’s “Printing Preferences” dialog box when it pops up (Look for “two-Sided Printing,” “Print on Both Sides,” etc). Also make sure that 'Flip on long edge' is selected.
The following are screenshots & links that demonstrate how to define 2-sided printing on different browsers and operating systems:
How to select duplex printing on windows 7 and above & internet explorer
How to select duplex printing on chrome
How to select duplex printing on iPad / iPhone
How to select duplex printing on mac firefox
How to select duplex printing on mac safari
How to select duplex printing on windows XP
Single Sided Printer
In order to print the card on 2 sides of the same paper, you will need to print 1 side at a time. As illustrated in the printing directions, it is recommended to mark the top right side of the paper, then print the first side of the card. Insert the same paper into the printer’s paper feeder with the marked corner on the top left side with the mark faced down, then print the inner page of the card.
If you experience problems with one-sided printing, we recommend printing only the first page (which contains the graphic elements on the outside of the card) and then writing the greeting text inside by hand.
Fold the card in the middle, and cut the card edges using scissors or a paper trimmer. Voila!