//get current user
$user = JFactory::getUser();
//verify if the user is logged in or not
if($user->id > 0){
	$app = JFactory::getApplication();
	$app->redirect('index.php?option=com_users&view=login');
}
else{
	$token = JHTML::_('form.token');
	$formLayout = str_replace('</form>', $token.'</form>', $formLayout);
}

//this will be the return URL once the login action was performed.
$return = JURI::root();
 
JSession::checkToken('post') or jexit(JText::_('JInvalid_Token'));
$app = JFactory::getApplication();
 
//construct login data
$data = array();
$data['return'] = $return;
$data['username'] = $_POST['form']['name of your username field'];
$data['password'] = $_POST['form']['name of your password field'];
$options = array();
 
if (true === $app->login($data, $options)){
	$app->redirect($return);
}
else{
	//invalidate username and password field if the login data was not correct, or the action could not be performed
	$invalid[] = RSFormProHelper::getComponentId("name of your username field");
	$invalid[] = RSFormProHelper::getComponentId("name of your password field");
}