I take it from the question about printing that you don't know how to mirror image them? Or is it that you don't know why you should do it???? If you don't know how....usually it's a function of your print properties window. When you tell your printer to print something, you usually get a window that lets you choose paper type, quality of print, etc. Some printers offer you t-shirt transfers as a paper type. If it does that, it will automatically mirror image the design for you when you select that type paper. Others offer a mirror image in other areas of the options. It varies. You'll need to explore your print options window. You don't have to do this if your design has no writing in it...as long as you don't mind it "looking" in the other direction when you're finished.

A lot of people do these on waxed paper or parchment - and then have issues getting them to release. I prefer plastic wrap...smoothed over the design and taped down so it stays smooth.

Be sure not to skip the part about gently smushing the finished design to eliminate any holes. "Gently" is the key word there. And, it also helps to add a final layer of icing to the design, just for strength. You don't have to be picky about that layer...all that's going to show is the first icing you laid down.

I like to do the fine details first...eyes, whiskers, fingers, etc. Much easier than trying to add them after large solid areas are filled in.

Be sure to let us see how your cake turns out!