Below is an example of a httpd.conf file, just copy and paste into your file, but change the paths:
ErrorDocument 404 /path/to/your/error/file/or/script
ErrorDocument 500 /path/to/your/error/file/or/script
ErrorDocument 403 /path/to/your/error/file/or/script
A very important point to note is that the paths should be the relative paths. This means that the paths would be what you would type on the machine to get to the files, not what you would type in the address box in a browser.
Full path would be http://www.yoursite.com/path/to/error/page
Absolute path would be /home/yoursite.com/path/to/error/page
Relative path would be /path/to/error/page