I think this might be quite simple, but i couldn´t figure out, how it works. I´m glad for a quick and short answer.
There are multiple ways to do it.
First, you can implement it using ACS commons if you have installed it. You can refer to the below URL for the same.
Second, If you don't have ACS commons installed in your AEM instance, you can follow the below steps.
1. Go to '/apps/sling/servlet/errorhandler'
2 create 404.jsp, default.jsp in this directory and you can write the code to render content on the error page.
Please refer the below article and you should be able to create a 404/500 error page using the ACS Commons Error Handler.