P̶l̶a̶c̶e̶h̶o̶l̶d̶e̶r̶ *Default* content in Custom Post Type TinyMCE editor

There is no way to add “placeholder” for TinyMCE, but you can set default content for newly created posts. All you need is just to hook default_content filter and return your formatted paragraphs:

add_filter( 'default_content', 'wpse8170_get_default_instructions', 10, 2 );
function wpse8170_get_default_instructions( $content, WP_Post $post ) {
    if ( $post->post_type == 'post' ) {
        $content=".... your content ....";
    }
    return $content;
}