In order to print both sides of a template, make sure that Print front and back of sheets is selected in step 4 Preview and Print.

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: