Creating a custom 404 page is very easy to do with Ink. First thing you need to do is just create a static page. The title of the page will be the main 404 message and the content will appear as normal content, in case you want to add something like links, icons etc.
Once you publish the page, you just need to select it as a 404 page, at the customizer.
Under the customizer, check for the “404 Error page” tab, click on it and select the page you just created at the dropdown. You should be good to go now.