I could easily see the sewn thread in the various signatures so I carefully removed the inner two pages which comes out as one sheet. That is two pages folded as one. I think it is okay to just tear out a couple of sheets at a time but I could see the threads well so chose this way.
Then I went thru the book picking four pages. I'd put a cutting mat behind the second and third page. Use a ruler to cut those two pages 1/2 inch from the spine.
Then I used gel medium to glue the page the two flaps and the two outside pages. To clarify I glued the outside of the 3rd page flap to the inside of page 4. Then I glued flap 2 to flap 3. I spread the medium on the outside of flap 2 and the inner side of page one and made them all fuse together by using my brayer. So you have 2 flaps glued inside 2 full pages. Perhaps I should have let these dry before doing more but I'm not that patient. I used wax paper between wet pages and dry pages and kept going. I found the gel a bit thick and perhaps I could have thinned it.
This is an example of the wax paper and brayer.
Then I put my sewing machine on the closed book to exert some downward pressure for the night.I removed the wax papers in the morning. It looked pretty flat but was also quite damp so I draped the book over a rung on my clothes horse for the day.
I saved all my pages for future projects. I also did some extra flaps. I want to add some loose pages later so I made two flaps side by side so I can stick in a loose page and glue it in between the two prepared flaps.
So far so good. Now I'm off to the art store in the city to ask all kinds of questions about gels, mediums, and glues.