Once you have applied the above configuration, you can test to make sure the <customErrors> section is correctly configured by requesting a URL like this from your site: http://mysite.com/pagethatdoesnotexist.aspx 

If you see the custom error page appear (because the file you requested doesn't exist) then your configuration should be setup correctly.  If you see a standard ASP.NET error then it is likely that you missed one of the steps above.  To see more information about what might be the cause of the problem, you can try setting <customErrors mode="remoteOnly"/> – which will enable you to see the error message if you are connecting to the site from a local browser.