Control Panel is on the roadmap fo the future.
For the moment, it is not possible to customize back-end menus, sub-menus, and CPanels.
- Create a new controller file, called maybe 'cpanel' (copy existing and rename / rewrite var and class names)
- Create a new VIEW directory called 'cpanel' in our example
- Don't create model because it is a menu and don't need database table. (If your menu is static)
- Create VIEW.HTML.PHP file
The display_XXX() function is the function to prepare objects for your layout. replace XXX with the name or your layout (maybe 'default' if your template file is 'default'.... In facts, it should be for this example)
- Create template file inside 'tmpl' directory. Name it 'default.php'
In this template file, try to copy how other components did it to get the same Joomla native style. Or copy the home back-end page.
- Then, if you want to acces this layout by default, find the way in the main file of your back-end. (com_mycomponent / mycomponent.php) in back of course.
- In the new controller file, add/modify lines in the display() function in order to load the right new template name.
Hope it can help you.