Within template:
	{if $preferFCK == true}
		{fckeditor assign="editor" name="new_text" x=600 y=600 text=$text toolbar="page"}
		{$editor->create()}
	{else}
		<textarea name="new_text" id="new_text" rows="10" cols="40">{$text}</textarea>
	{/if}

But first, plugin must be registered when smarty object is created:
	require_once($smarty->_get_plugin_filepath('function', 'fckeditor'));

And the plugin itself:
	<?php
	/*
	 * Smarty plugin
	 * -------------------------------------------------------------
	 * File:     function.fckeditor.php
	 * Type:     function
	 * Name:     fckeditor
	 * Purpose:  Creates a fckEditor instance on demand
	 * -------------------------------------------------------------
	 */
	function smarty_function_request($params, &$smarty)
		require_once(DOC_ROOT .'/lib/FCKeditor/fckeditor.php');
		
		if (isset($params['name'])) {
			$smarty->trigger_error("editor name missing", E_USER_ERROR);
			return;
		}
		
		$data = new FCKeditor($params['name']);
		
		$data->BasePath		= DOC_ROOT .'/lib/FCKeditor/';
		$data->Width		= (isset($params['x']) ? (int) $params['x'] : 400);
		$data->Height		= (isset($params['y']) ? (int) $params['y'] : 400);
		$data->ToolbarSet	= (isset($params['toolbar']) ? (int) $params['toolbar'] : 'Default');
		$data->Value		= (isset($params['text']) ? $params['text'] : '');
		
		if (isset($params['assign'])) {
			$smarty->assign($params['assign'], $data);
		}
		else {
			return $data;
		}
	?>