Here’s how it works. You choose a size for your book, format your Word manuscript to fit that size, turn your Word doc into a PDF, create some cover art in Photoshop, turn that into a PDF, and upload it all to the self-publisher of your choice.
If you succeeded in formatting everything correctly, you will get a book proof back within a couple of weeks (or sooner).
After you officially publish your book, you can make changes to your cover and interior text by submitting new PDFs, though your book will go offline (“out of stock”) for a week or two.
Caveat: Creating a book that looks professional and is indistinguishable from a book published by a “real” publishing house is very difficult and requires a minimum investment of a few thousand dollars (when all was said and done, usually around $7500, which includes about $2,500 in marketing costs).