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.
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: