$custom_tabs = array(
  array('title' => 'Tab 1', 'contents' => 'Content for tab 1'),
  array('title' => 'Tab 2', 'contents' => 'Content for tab 2'),
);
echo drupal_render(quicktabs_build_quicktabs('my_custom_tabs', array(), $custom_tabs));

$tabs = array(
  'tab_1' => array('type' => 'freetext', 'title' => 'Tab 1', 'text' => 'Content for tab 1'),
  'tab_2' => array('type' => 'freetext', 'title' => 'Tab 2', 'text' => 'Content for tab 2'),
);
echo theme('quicktabs', array(
  'qtid' => 'my_custom_tabs',
  'tabs' => $tabs,
  'style' => variable_get('quicktabs_tabstyle', 'nostyle'),
  'ajax' => FALSE,
));

Quicktabs 3.x