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.
The simplest element is the alert element and you can access it here and start playing around.