Yin Gao didn't write backwards. He started the manuscript with raw rice paper and thick ink, and the words were clear and round. Then cover the Yin Shi, stroke the edge of the paper on the seal, and hold it tightly with your left hand. At this time, I saw the reverse side of the rice paper. Wet the printed surface slightly, scrape it repeatedly with the back of nails or horn pieces, and finally remove the draft of rice paper, and the "reverse word" will remain on the printed surface.
You can also print directly with a brush. Type out the manuscript with rice paper first, turn it over and put it on the table (the words on the front can be seen clearly on the back of rice paper), and copy it on the printed surface like a copy. After copying, look in the mirror.