This article covers two ways to set up 404/500 error pages, but there are many additional methods available.
1) Create a .htaccess file within your public_html directory, containing the following two lines:
ErrorDocument 404 "/404.html" ErrorDocument 500 "/500.html"
These directives send 404 and 500 errors to corresponding .html pages within your webspace. You will need to manually create the .html pages, and configure them to suit your requirements.
2) Within your eXtend control panel, select "404/500 Error Page" from the 'CGI Scripts" section and install to a directory of your choosing. This will set up the .htaccess file and .html error pages for you. Errors are instead sent to a cgi script, which in turn redirects users to the corresponding htmlerror page.