For an overview of the experimental editor for AMP Stories, please see: https://github.com/ampproject/amp-wp/wiki/AMP-Stories-Experimental
Note that you will need the latest version of the Gutenberg plugin installed, since the AMP Stories editor uses bleeding-edge features of WordPress's block editor which are not yet in a WordPress core release.
To test, please install the
amp.zip build below by uploading it in the WordPress admin. Once active, enable the AMP Stories feature from the AMP admin settings screen.