Print Two-Sided; Print Both Sides; Double Sided Printing

When you are designing a two-sided project, be sure you have chosen the correct template to print both sides.  If you need to change templates after you've designed one side, use the Change Template option on the top right of the Customize screen to transfer your design to the two-sided template.  Use the Navigator to switch between front and back. When you're ready to print, if you have a printer that offers two-sided printing, you can test on a plain sheet of paper to be sure it will print in the right orientation.
In order to print both sides of a template, in the Preview & Print step, select Show Print Options on the right side and select Print front & back of sheet(s).

Once you have your printer window open, you will want to have two-sided selected.  If you encounter issues with alignment using the two-side option, print one side at a time so you can better control the feed.
If your printer does not offer two sided printing, or if it prints the back side upside down, it's easiest to print the fronts first and then the backs.  In the dialog shown above, choose Print front of sheet(s).  This will generate a pdf for the front side.  Print that pdf on a plain piece of paper as a test.  Leave the pdf open and go back to Design & Print Online to select Print back of sheet(s) to create a second pdf.  Re-feed your test paper and note which direction you try.  If the back printed correctly, you can print on Avery products. 
Check out our Help Video on Two-Sided Printing:

Note: This printing process is different for Tent Cards