Tools » Custom Error Pages. . .

Setting Up a Custom Error Page:

On our servers, you have the option of creating your own error pages; however, due to space limitations here, we're only going to cover the Error 404 (File Not Found error page). The procedure to set up the others are the same.

You'll need to create two files to accomplish the task. The first file will be a plain HTML file that you can make look to your liking, using graphics, backgrounds, text color, etc., just like you would any other page.
The second file you'll create is an access file which makes your custom error page load when visitors click on a page link that is no longer valid, or when they type in a URL that is not there. This access file is sometimes referred to as a "dot file" because the file name looks like this: .htaccess -- note there is no file name before the dot.
Create your .htaccess file using a plain text editor such as Notepad. Open a new document in Notepad, and type a line similar to the one below.

ErrorDocument 404 http://www.yoursite.com/filename.html
Save your file as .htaccess and upload both your html page and your .htaccess file to the Main Directory of your site. Your domainname-www directory. Be sure that you upload both files in ASCII format, and not binary format.