Add a custom widget box on the sidebar of chosen page or post types to provide instructions to the content editor.

wp-add-cms-instruction-box.php

add_action( 'add_meta_boxes', 'add_side_instructions_box' );
 
function add_side_instructions_box() {
   foreach( array( 'custom_post_type', 'page' ) as $customPages ) { //list which posts you would like this to appear on
       add_meta_box(
           'project_side_instructions',         //meta box id
           __( 'Helpful Hints', 'site_domain' ),     //meta box title
           'inner_side_instructions_box',        //callback to function with box content
           $customPages,                 //which pages the box appears on
           'side',                     //area of page where the box appears
           'low'                     //box priority on page
       );
   }
}