PHP Reference - mithra62/ee_debug_toolbar GitHub Wiki
The Debug Toolbar (EEDT) comes bundled with functionality to ease the pains of creating custom panels and extensions.
There's no better example or documentation available than the bundled extensions. Be sure to check out how they're put together to get a full understanding of how things work.
In order to use these PHP classes you'll have to ensure that the EEDT files are available to your extension by adding the below code to your constructor.
<?php
ee()->load->add_package_path(PATH_THIRD . 'ee_debug_toolbar/');