Open the book to the first page to be folded. Line your ruler up on the edge of the page opposite the spine and make a mark at the START and END points for that page. Line your template up at the top mark and the edge of the page with the slant going towards the top corner at the spine. Mark a line along the template. Flip the template over and repeat for bottom mark.
Fold the corners of the page in along the marks. First sheet done, only a few hundred more to go. Repeat for each sheet, if you lose your place simple look at the page number you are on and find it in the excel table. Continue until all pages are finished. Adjust looseness of the book until the image looks right.