You might find that inkjet print doesn't work very well on card that's already been printed on. (You could end up with a faint or smudged result).
However if you want to give it a go, measure the opened card. Then go to a word-processing program (such as Microsoft Word or OpenOffice Writer) or a DTP program (such as Microsoft Publisher or Serif PagePlus SE) to create a blank document (using the 'custom size' settings) which is the same size as the card.
Use a ruler to work out where the text needs to be on the card and then type into the blank document at the relevant place. (If you're using a word-processing program you'll need to keep hitting the Return key until the text is far enough down the page. If you're using a DTP program you can simply create a text box in the appropriate place).
You should then be able to print correctly onto the card (although I suggest trying it on a blank piece of paper, the same size as the card, first).