Do I use the Leanpub editor to make Markua? How do I get into the Markua beta?

The decision of "is your plain text considered Leanpub Flavoured
Markdown or Markua" is just a checkbox you click at
https://leanpub.com/YOURBOOK/edit_manuscript_format

Learn more here:
http://help.leanpub.com/author-help/how-do-i-write-a-book-in-markua-today-whats-the-current-status-of-markua