To do this, you start by setting up the basic details of a new book by clicking the blue button here: https://leanpub.com/authors. We recommend using the option to write "In a Dropbox folder on my computer", but you can also choose the option to write with GitHub, Dropbox or in your browser.
Once you have your new book set up, you will see a "book tools" menu on the left. Click on "Import" and then select "WordPress" and follow the instructions there.
Please note that you will probably have to do some curating of the book after you import to fix up the formatting etc. If you encounter any issues or if anything's unclear, please let us know.