The title is a bit over dramatic but, per the title, if you could contribute with one piece of knowledge to a book that every single individual should learn from in order to kickstart a civilization, what would be yours?
My personal choice would be the process of soap making, from scratch.
It’s hard to choose “one piece” of knowledge, so I would try to persuade whoever is writing the book to include the time traveler’s cheat sheet. https://i.imgur.com/O6vSrvq.jpg
I didn’t make it, but includes a lot of information that didn’t have an intuitive path to discovery, but a lot of practical benefits for humanity. If I were to add to it I would try to include at least descriptions of a few other things: