How do I add own buttons in Typo3-Backend to call extension method

could anyone tell me, what's the way to add own buttons in the backend of Typo3? It should appear there:

THX in advance!!

TYPO3 6.2: Use the file .../typo3_src-6.2.4/typo3/sysext/backend/Classes/Controller/BackendController.php and its method addToolbarItem.

/**
 * Adds an item to the toolbar, the class file for the toolbar item must be loaded at this point
 *
 * @param string $toolbarItemName Toolbar item name, f.e. tx_toolbarExtension_coolItem
 * @param string $toolbarItemClassName Toolbar item class name, f.e. tx_toolbarExtension_coolItem
 * @return void
 * @throws \UnexpectedValueException
 */
public function addToolbarItem($toolbarItemName, $toolbarItemClassName) {
    $toolbarItem = GeneralUtility::makeInstance($toolbarItemClassName, $this);
    if (!$toolbarItem instanceof \TYPO3\CMS\Backend\Toolbar\ToolbarItemHookInterface) {
        throw new \UnexpectedValueException('$toolbarItem "' . $toolbarItemName . '" must implement interface TYPO3\\CMS\\Backend\\Toolbar\\ToolbarItemHookInterface', 1195125501);
    }
    if ($toolbarItem->checkAccess()) {
        $this->toolbarItems[$toolbarItemName] = $toolbarItem;
    } else {
        unset($toolbarItem);
    }
}

Do it the same way as the opendocs system extension does it.

if (TYPO3_MODE === 'BE') {
    // Now register the class as toolbar item
    $GLOBALS['TYPO3backend']->addToolbarItem('opendocs', 'TYPO3\\CMS\\Opendocs\\Controller\\OpendocsController');
}

See the file .../typo3_src-6.2.4/typo3/sysext/opendocs/Classes/Controller/OpendocsController.php:

class OpendocsController implements \TYPO3\CMS\Backend\Toolbar\ToolbarItemHookInterface {