To create a custom 404 page in WordPress, you can follow these steps:
1. Open your WordPress dashboard and go to Appearance -> Theme Editor.
1. From the list of files on the right-hand side, select 404.php.
1. If there is no 404.php file in your theme, you can create it by using a code editor like Notepad++. Simply create a new file and save it as 404.php.
1. Add the following code to your 404.php file:
```
get_header();
?>
1. Customize the content and layout of your 404 page as per your requirements.
1. Save your changes and your custom 404 page is now ready to use.
Note: It is recommended to use a child theme for making any changes to the code of WordPress theme files. This prevents any loss of modifications in case the theme is updated in the future.