The back cover MUST have a synopsis of your book so that they are then given extra incentive to open it up. It's a luring-in process. You have to get them to pick it up in the first place ... but then you have to get them further drawn into what you are offering. That back cover is critical.
For online sales the back cover is less critical because the blurb is right there in front of them. Still, some people like to see the back cover in image form as well.
Here is how I lay out my romance novels:
The front cover has the image that grabs at people and the title in large letters. It's important that the reader understands right away what the book is about. Don't go for obscure fonts and hard to read things. Make it obvious.
Then on the back, the ONLY important thing on the back is that they can read the words there. Everything else is secondary. Make those letters easy to read, so they are happy and comfortable in reading your description. Make sure the contrast is well done so that it is an easy thing for them to do.
These words are your "sales pitch". Think of what YOU would like to read about a book, that would draw you into reading it. Make it interesting. The characters have to seem really intriguing. The situation has to seem compelling. Test your sales pitch out on family and friends and ask for suggestions. Write it and rewrite it.
Make sure you leave room at the bottom for your ISBN bar code! Without that code, pretty much no bookstore will be able to sell your book. Many self publishers will put that on for you - but some will not. Check that out so you know how to design your back cover.
In terms of the overall template, every modern self-publisher should now provide that to you as part of the process. You should be able to enter the book size you've chosen, the page count, and the publisher should provide you with the base template on which to create your image.
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