Using this template you can design a new JFC (Swing) internal frame that can be placed onto a JDesktopPane to provide an MDI user interface.

Typically you would create an internal frame class (or several) containing display elements for each individual frame; treat it just like JFrame. Then create an application JFrame (or JApplet) with a JDesktopPane on it. The internal frames should be added to the desktop pane using code, not visually in the Form Editor.