This article explains you how to post a pretty personalized page 404.
This note applies to Linux lodging.

Prepare initially the page to be posted in such a case and save it in
the root directory of your site. For example, let us call it 404.html.
Check that it is posted correctly while going with your navigator on
the page (Ex: http://www.monsite.com/404.html)



It is then enough to put, in the root directory of your site, a file
named htaccess which contains only one line:
ErrorDocument 404
/404.html You can then test while going on a non-existent page of your
site (Ex: http://www.monsite.com/segfdhsf).

Caution!

If you wish to make so that this page is dynamic (for example
that it is made in PHP), it is necessary well to take guard so that it
does not reach a data base nor any other task which requires
processing time. Indeed, your site could quickly leave in loop if a
file missing but required often (for example the favicon.ico) owed, at
each request, to take resources machine.
It still should be
known that this page 404 will be generated (and sent) for each request
which does not succeed; including thus the favicon, missing images,
requests of robots which would seek pages which would not exist any
more (and their images), etc.
Be thus particularly attentive
in the case of a page 404 dynamics and, where necessary, do not
hesitate to address to you to us for further information.