Edit and include content block in 404 page not found

I am using Concrete5 version 8.5.4 and PHP version 7.6.
I want to customize my 404 page not found with images and contents but I am unable to find out the way through which I can do it.
Help me to figure out how to do.

The ‘Page Not Found’ is a single page and is located in the concrete/single_pages folder, it is named ‘page_not_found.php’.
To modify it you should first copy it to application/single_pages and then find it the sitemap by including system pages,