Some printers have a "mirror" or "t-shirt transfer" printing option
built into their printer driver... in Word, try clicking on File | Print
| Properties, and see if that option exists for your printer. If you
have Adobe Acrobat (the full version, not just the free reader), then
the Acrobat PDFWriter printer driver has this option, so you can make a
mirror-image PDF file and print that onto your transfer paper.
If you're trying to mirror text, see
http://www.gmayor.com/rotate_text.htm or
http://www.word.mvps.org/FAQs/DrwGrphcs/UpsideDownText.htm .
If you're trying to mirror an image (like a photo or a drawing), format
the image as "float over text" (Word 97) or with a text wrapping style
other than "inline with text" (Word 2000-2003), and then flip the image
using the commands from the Drawing toolbar.
