Let notfound.org make a better use of your 404 error page on Drupal

1. Visit notfound.org and click on "Yes, I'll make a better use of my 404-page"
2. Register your domain and copy the html code generated.
3. Create a Drupal content. A "basic page" it's ok.
4. Insert "404 - Page not found" as title and paste the code copied from notfound.org
5. On publish options, untick Promoted to front page

Change your 404 error page on Drupal 7

1. Goto configuration and then Site information. admin/config/system/site-information
2. Search field with the name default 404 (not found) page and enter the node/nid  of the content created.
3. Test it by visiting a page that does not exist.