Q: How do I order square prints?

Where are you seeing square prints listed on Apple's site?

The Print Products pages is only listing these sizes:  Check the list for your country:

http://support.apple.com/kb/HT3491

Are you wanting to print the prints yourself or have Apple print them.  With iPhoto 11 start the print process, select Custom layout and a 1:1 aspect ratio in the menu at the bottom of the window:

However, there are no options for square prints from the Apple Print Store.

OT