Need help making postcards (formatting)

I have this great idea to put little postcards advertising my book in with my Christmas cards when I deliver the paper next week. My issue? I don't know how to format so the front and back side line up. Is anyone here proficient enough with Word/open office to format something like that for me. I want 9 per page so I can cut them out and put them in the paper (2.5x3.5). Thanks in advance.