Can anyone tell me how to do this?

Put waxed paper over your image, trace it with piping gel onto the image, let it sit for a minute until if feels a little tacky and not runny. Turn it over onto your cake and press along the tracing.

This will be a mirror image of your pattern. If you want the same image, I take a thin black Sharpie marker to trace the image so it comes through the back, and then put the waxed paper on the back side.

Or, you can scan the image and mirror, then print.

I normally use thinned black icing instead of piping gel, the pattern is easier to see and does not spread like the piping gel does. The color usually transfers but the icing normally stays on the waxed paper, if the icing does come off, you can just take a toothpick to lift it off, but the color will still be there, just use a small round tip.

HTH