layout and paginate a saddle stitched bookletpd

An InDesign project. All pages were created by our client, but they don't know how to paginate for print. They are currently on single pages in a pdf file. The pages are set up on 9.5 x 12 with marks and bleeds. We need them to be set up 2 per page in an InDesign printer's spread file. The finished printed size will be folded to 8.5 x 11, after trimming to the bleed.