Creating elements for JD builder is really simple & straight forward, There are more than enough fields that let you create almost any kind of layout imaginable, view list of fields here .
You can easily copy one of the existing elements to start, to view all the available elements, click here.
While writing the element, always follows Joomla’s coding standards and best practices.
The strucure of an element is something like this.
element.xml (xml file with element info and configurations options that will appear in the backend) language/en-GB/en-GB.element.ini (Language files should be used in order to offer localization when and if needed) tmpl/default.php (The primary layout file that is used for element layout on the frontend) tmpl/default.js (The primary javascript file that is used for rendering the layout in element's live preview)
The simplest element is the alert element and you can access it here and start playing around.