This plugin is designed to work with as many themes as possible. It uses the WordPress Template System to detect and handle 404 errors. If your theme makes use of this Template System this plugin will work properly. If it does not the plugin offers a so called Compatibility Mode, which uses its completely own method to detect and handle 404 errors. If this plugin does not work properly, try to activate Compatibility Mode.
How to Create a Custom WordPress 404 Error Page
Chances are you have created your website and checked multiple times to be sure that every link leads to somewhere. Eventually over time you may forget about a link that leads to a post you removed, or possibly changed the name to. Even if a visitor is searching for a page directly and they don’t get it right, then they will be taken to the error 404 page.