Write the correct 404 erro page or codes

I think everybody can make a 404 erro page for his website,but do you know how to write it correct? do you know whether the 404 erro page return the correct erro code? do you know whether it is good for SE(search engine)?

First,you must set up a web page and name it like 404.htm/html

second,you must customize your default 404 erro page,use the 404.htm/html you made

third,you should check the returning code of your customized 404 page,and make sure it return 404 erro code,not 301/302.

by the way,you can use software named “Fiddler” to check your web page status.

Enjoy it!