How do I create a custom 404 page for my Blogger custom domain?

Blogger doesn't have a direct setting for creating a custom 404 page. However, you can achieve this by creating a new post, designing it to look like a 404 page, and then using JavaScript in your theme's HTML to check for specific conditions (like the absence of a post) and redirect to this designated 404 post. A more advanced method involves editing the theme's XML to conditionally display content.