Make your 404 page.
Publish it.
Add this code to your .htaccess file in your public_html folder.
ErrorDocument 404 /404.php (or use 404.html if the page is html)
You will need to generate a .htaccess file in your BV Folder.
Using the FTP Manager in your web builder you can then drag the file into your public_html file.
If this is for an addon domain make sure that you drag the .htaccess file to the correct folder/domain.
As an added precaution copy the old .htaccess and store it in case you make a mistake whereby you can drag the original back to its proper place.

After you do this procedure once you will be able to do it on your sleep or when your supposed to be sleeping!