For all sites, we have a default 404 Not Found page that is very simple and takes its aesthetic from what you've already set up in your site's Global Settings .
Example:
But let's say you want to customize this simple layout, and maybe even monetize it. (Go you!) Here are the steps to get this done:
1. Go to the Layout & Design tool in the left-hand navigation menu.
2. Choose the 404 page.
Once you're in the 404 page, you can style and customize anything you want in it. This page — like any of the other Pages found in the Layout & Design tool — can be rearranged with our Row and Column elements.
Here's an example of a customized 404 page where the Top Bar has been removed, some suggested posts have been added, and a Revcontent module has been placed at the bottom of the page: