Posting backwards is for a good omen, and posting directly is also acceptable.
Since ancient times, people have thought that peach wood has the function of exorcising evil spirits, and then painted two immortals on peach wood, which is called Taofu, hoping to ward off evil spirits and welcome good fortune in the new year, so this Taofu is the earliest prototype of Spring Festival couplets.
in the five dynasties, couplets began to appear on the "Taofu", which were still two mahogany boards, but instead of painting the door gods on them, a couplet was written on it, because couplets can more directly express the good wishes of the people to bid farewell to the old year and welcome the new year.
in the Ming dynasty, people began to write peach symbols on red paper, because the mahogany board was too heavy after all, and then writing Spring Festival couplets on red paper was not only very convenient to paste, but also very festive, so the people in the Ming dynasty called peach symbols "spring stickers".