Hello,
We don’t have documentation for this, unfortunately.
Anyway, it’s very simple. Once you enable builder in XStore > Email builder you are redirected to page where you could create email templates or edit pre-created https://prnt.sc/12j3gfi
You can choose/change the email type in the right corner, at the left side you have elements that you can drag&drop to email area and style according to your needs https://gyazo.com/657d545eeef52522ade7bf333689a68f
Regards