There are many tutorials on line for crating a dreamweaver template including this one:
The basic steps are to create a page that represents your site design. you can do this using CSS Sculptor.
you then convert it to a template by going to File -> Save As Template.
once the design is converted to a template, you can add editable regions by going to Insert -> Template Objects -> Editable Region.
once you have a template created, see the "Using themes in Data Bridge and Design Extender" tutorial on the design extender support page for details on converting the template to a theme: