how do i format and print a double sided greeting card in pages

The Pages app will have two pages of output.

How to print that depends greatly in the capabilities of the particular printer.

If the printer does not support duplex (double-sided) printing, then you’ll have to print one page, and add the printed page of paper back into the input tray to print the second page.

(Getting the paper orientation in the paper feed tray right usually takes a couple of tries, so don’t use “good” paper to start with.)

If the printer does support duplex printing, select that option when printing.

The duplex printing option is available in the fully-expanded print dialog, when the printer supports duplex printing.