You're welcome Nutty!
can anybody tell me how to customize it
It is really easy. Just create a page on your site, customizing with the content you want included. Name it whatever you want, for example missing.html. Then your host can set it for you, or within your cpanel there should be a function to enter the url of the page you made so it displays anytime someone gets a page not found error. Once you've done that, enter a url that doesn't exist on your site to confirm you are getting the custom page you intended. Let us know if you need more help.