All you need is to add methods like these and modify $params['data']

"Render" events are called once and the content is saved to DB
RenderItemAsHtml
RenderItemAsXml
RenderItemAsText

"Display" events are called on each page view
DisplayItemAsHtml
DisplayItemAsXml
DisplayItemAsText

Example

	/**
	 * Event handler: Called when rendering item/post contents as HTML.
	 *
	 * Note: return value is ignored. You have to change $params['data'].
	 *
	 * @see Plugin::RenderItemAsHtml()
	 */
	function RenderItemAsHtml( & $params )
	{
		$params['data'] = 'TEST['.$params['data'].']TEST';
	}



	/**
	 * Event handler: Called when displaying item/post contents as HTML.
	 *
	 * Note: return value is ignored. You have to change $params['data'].
	 *
	 * @see Plugin::DisplayItemAsHtml()
	 */
	function DisplayItemAsHtml( & $params )
	{
		$params['data'] = $params['data']."\n<br />-- test_plugin::DisplayItemAsHtml()";
	}