If I am understanding what you are wanting to do, create an A3 size page, put any guidelines on it that you may need. Create two pages (assuming it is two-sided). Use one of the pages as the outside, one of the pages for the inside.
For the outside (the "cover"), typically the right-hand side is the side one sees first, the left-hand side being the back.
Communicate with the printer (the print service that is) about it being a folded job.