There are many ways to add end pages to your book. This video shows how to attach folded end pages and a headband to a multi-signature text block. The end page is glued with the folded peak meeting the spine of the book (images below).
Note: The video says you'll have to cut your end pages for the 6 x 9 but I actually provided those in your kit. As always, you're free to replace with a different paper if you'd like.
Use the same method to attach end pages to your other multi-signature text block.
Here is what the attached end pages should look like. Notice the "nice" side is on the inside of the folio. This page will act as a hinge that attaches directly to the cover.