The page size in your file needs to have a tiny bit of width and height added to 3 of the sides (outer edge, top, and bottom) that eventually get trimmed off to your final page size. This is called a bleed, and should not affect the size or placement of the content within the page. It only makes the overall page size set up in your file bigger than your final printed size. For example, say you had a 4x6 photo mounted onto a piece of paper that is 5x7. But your picture frame is 6 x9, so the 5x7 paper won’t cover the entire background in the frame. So you mount the photo onto a 7x10 paper, which you then trim down to 6x9. Now your paper covers the background entirely. This is essentially what you are doing for your pages.
And then there’s Step 3: Gutter margin. The fourth edge where the two pages meet where bleed margin was NOT added, faces into the gutter, or the binding, of the book. But if the image content goes all the way to this edge, that part of the image will get lost INTO the gutter, because a portion of paper is used to make the binding. So the image must be sufficiently outside this gutter margin to avoid losing part of the image to the binding. However, if the image is moved too far out, there will be a blank area in the middle of what is supposed to be a continual image. Personally, I didn’t want a gap in a single page or a spread image, so I decided to set up the page where my content would fall just beyond the gutter margin. Then I’d lose a tiny amount of image on purpose, but would ensure that no gap in the image spread would occur.
Another month of my life would be devoted to getting to this stage. NOW was I ready to go?