It can be tricky to "design" a spine for self publishing that ends up in the actual spine location :). The problem is that the spine's width varies based on how many pages you have. If you have 10 pages, the spine is much thinner than if you have 1,000 pages. You need to design your cover to take that into account.
Every self publisher I've used will provide you with a template to use, and tell you how wide the spine is. What if you need to make adjustments, though? What if you decide to add in twenty more pages? How can you get that spine to adjust properly?
Here is how I handle it.
First, in my template I always put a guideline mark at the "center of the book spine". Everything works outwards from that center point. I also put in guideline marks for the top edge and the bottom edge of the book. Your book is never going to get "taller" :). It will always be a certain height. Your cover, therefore, will always be that height.
The way covers work in printing is that they are a single wrap-around piece of thick paper. The printing on that cover, if you laid a book out flat, would have the back cover on the far left, then the spine in the middle, then the right cover on the far right.
Therefore what I do is first design a front cover image, of the set size. That front cover image will never change in size regardless of how many pages you have. If your book is 5" x 8" in size, that will always be the cover size.
The same is true for the back cover. It will always be an exact size. So I make up two JPG images - one for the front cover and one for the back cover - that are set in stone and finalized.
Now, in PhotoShop, I take my template that has guidelines for the top of the book, the bottom of the book, and the center of the printing run. I bring in my "spine background" and put it in the center of the layout, on the bottom layer. I always have far more spine background than I need, so that it will work regardless of how many pages I end up with. I put on the title, the author name and the imprint logo using my "center guideline" to align them down the center of the spine. I have the top edge and bottom edge guidelines to help with the alignment.
Next I have to figure out where to put the front and back covers in relationship to that spine content. I make a new layer and select an area the width of what the publisher told me is proper for my page length. If I have 300 pages and the publisher says the spine should therefore be .75", I make a selection area that is .75" and a random height, maybe 2". I then fill that area in with black. Now I have a black rectangle sitting on my page.
When you're working with shapes in PhotoShop, you can always see the "center point" of that shape when you move them around. So now I move that black rectangle and I can see - visually - when the center of that black rectangle is at the center of my spine.
Now that I have something to guide me, I can slide the right (top) cover in to the left until it abuts that black rectangle. I can also slide the left (back) cover in to the right until it also abuts that black rectangle. I now have a spine that is perfectly .75" in width and the two covers appropriately up against it.
I delete that black rectangle before I save my final version.
I know this is a bit confusing, but try taking out any paperback book you have and gently flattening it so the cover is one piece. You'll be able to see how this should work. It might take a few test runs before you get it just right, but it's worth it!
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