Create a custom error 404 page for WordPress

Step 1. Go to the directory /wp-content/themes/twentyfifteen/
Step 2. Create new file 404new.php
Step 3. Delete or rename the file 404.php
Step 4. Open files index.php, search.php, archive.php, image.php, replace command get_template_part( 'content', 'none' ); by command include ('404new.php');, as below:
Line 51.     // If no content, include the "No posts found" template.
Line 52.     else :     
Line 53.     include ('404new.php'); // get_template_part( 'content', 'none' );