Suggestion:
Why do you need software?
Do you have Microsoft Word? If so, use that. If not, Google for OpenOffice.
Set up your wording, load the paper into the printer, and print.
If you're concerned about whether the wording will overlap the sections, use the percentage slider/dropdown menu (depending on what version of office you're using) to make the size on the screen the same as the size on the paper. You'll be able to visualize where on the document the designs are, and place your text a safe distance away.
If you're not sure which way to load the paper, that varies based on the printer. To test it, just get a plain sheet of paper, and make a mark on it. Put it into the printer with the marked side up, and print something on it. If your text shows up on the side with the writing, load your paper design up. If it shows up on the other side, switch it around.
If your issue is paper sizing, find your printer options in whatever program you have chosen, and change the paper size accordingly. If your paper is particularly thick, you may need to look at the packaging to determine whether you should change any other printer options. Then, find the "page setup" or "document size" (this, too, varies based on your version of Office) and change the dimensions there, as well.
Don't forget to print a test page on your invite paper before you start it printing them all — it can help to avoid mistakes.

{ 1 comment… read it below or add one }
How big are the invitations?