When you first open your book, it is displayed like you were looking at a book. To design your cover as a spread, click on the green Front Cover ribbon on the right side of the page.

Next click on Edit Spread.

Now you will see the cover as a complete spread. You can drag an image and resize it to be spread across both sides of the book.
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If you have a perfectly sized image, you will simply drag the image onto the page, click on it, and then click on the bottom right resize handle and resize it across the page to fit both sides of the cover. If the image is not exactly the right size, you might find it useful to hold down ctrl while resizing so the image does not have to maintain the original aspect ratio. This way, the cover image will be cropped to fit in the given space.

If you are working on a hard cover, you will have some space in the middle where you can place text on the spine. Be careful to keep the text only in the white area of the spine or it could be cut off or bleed into the front/back sides of the book.

If you are going to be printing multiple covers, you will click on the green Front Cover menu button to open up a sidebar. The other cover options will show as edit buttons with the name of the cover type. Clicking on this button will switch to editing the other cover format.

After clicking on the other format, you can start designing the second cover as well. It can designed to be completely different or the same with minor tweaks depending on what you want to accomplish.