In the same project I created a new page, using my own custom dimensions (1000x1000).
Switching back to the original page, I then copied all the assets from the first cell of my first page. (You can Shift+Click to select multiple objects in the web app, or choose Select Multiple in the mobile app.)
I switched to the new page and pasted the objects onto the canvas. I did have to reload my background image as that element could not be copied.
I positioned my objects so they were a close match to my original design (Express does not offer the same kind of pixel-level control like Photoshop, Illustrator or InDesign - there are no rulers, just Smart Guides). The challenge for me was the connecting arrow; because it sat between the main graphics, the arrow had to line up perfectly from image to image so that the carousel looked seamless.
Note: there could be a certain amount of eye-balling and you might have to download your series to check that everything aligns properly I know I had to do this with my first carousel and ended up making minor corrections to the position of my arrow connector.
Then I repeated the process with the second cell of my original design. Now, this may sound like a tedious process, but here's where the fun begins.
After positioning all my elements for my first how-to step, I simply clicked the Duplicate Page tool. This icon is found below your canvas area in the web browser or at the top of the screen in the mobile app. Duplicating the page makes it easy to build out the basis for your other carousel images; you simply edit each new duplicate to create your story.