This is the pattern, template. All dotted lines should be creased, all full lines should be cut.
The back side of B is glued to the backside of 1. This then is glued to the backside of 2. A is sticking out right now. Further: backside of 3 to 4, 5 to 6 (it goes down now). Until 16. Then glue the backside of C to 18. Don't close 17 to 18 yet ! First fold the whole book together. Then take flap A, fold it around the backside of the book, which has now some thickness, and insert the rest of A in 17-18. Then close backside 17 to 18. Book is done !
Some white glue could first be applied to the backside of the folds before inserting A in 17-18. But it is not really necessary. All the folds could be glued with a simple glue stick. It is even not necessary to glue the whole page, just a few strokes.
It's clear that B and C are only there to reinforce the cover pages.
There is another possibility with this template, even simpler: skip B and C, and insert A fully to 17-18. You then get a "star" with 9 sides. It can't be folded into a book any more, but it can be folded flat. Nice for taking notes, and then you hang it in the air.
The squares are all 5 to 5 cm, so this template fits on a (European) A4. All pages are on one side too, ideal for photocopies. Certainly the "star" can also be made out of thick drawing paper for painted artist's books.