Set this up from your DocumentRoot if you use Apache. In my case, I have this folder inside of /var/www/html. I've included some of the important bits from my Apache config files as well. We use ssl so you have to catch the 503 in both places.
cganote/custom_errors
This is mostly for me to keep my ErrorDocuments straight. I have a few galaxies that benefit from having 503 catch pages!
CSS