Inserting pages into a document by definition will change the layout as it will shift pages that may have been designed to be a spread but that is just the law of physics. Two objects cannot be in the same place at the same time. That being said I would build the document as facing pages as in the screen shot below:
In the first picture I have created a facing pages document where I created three master pages. The first is a single page for the cover and the second and third are spreads created for document pages and ad pages. I've included auto page numbers for each on the outside of the spreads. In the second picture I inserted a right page master from the ad pages (Note that the original page three is now automatically page four). I repeated the process in the third picture with a left page from the ad page master. Notice in picture two that there is no back cover. You must make sure that for a saddle-stitched brochure that the number of pages is divisible by four and that with a facing pages document the last page is a single page. When you design the ad page masters only include the page numbers since you will want to place the content of them on those pages manually.
1 person found this helpful
It just occurred to me that your question might have been even more basic than at first occurred to me. If you're asking how to insert the master pages then you can do that by using the Pages Panel pull-down menu "Insert Pages" and choosing the options that are shown in the screen shot below: