Well, this is an account of the way I understood the retchoso binding method, as described here
Maybe someone more expert than me can correct me if I’m wrong, otherwise you can find an explanation of rather simple way to bind a multisection book that open very flat that can be useful to bind your own notebook.

I started preparing the signatures and making the holes for sewing. Then I started sewing from the central signature in that way (red dashed lines are when the thread is inside the signatures)

Then when I reached one side I made returned toward the central signature and continue sewing until I reached the back of the book

And I returned back to the central signature where i stopped making a knot

When I finished with the upper part of the book, I made the same thing, with another thread, for the lower part. When I reached the end in the center of the book, I close the thread, like the other, with a knot and then with another pair of knots I joined the upper and lower threads (I don’t know if you really need to do that.

I show you the resulting book (not so nice, I’m not very careful when I do that kind of things):