hosting script file in subdomain

Hi all, i read once that we can put our script file in subdomain like http://script.yourdomain.com to speed up page load, but unfortunately i did not find any clue on how to do it. I already have a subdomain for my image files and it works very well. i found the article here but there is no tutorial about how to do it in script file.
i also intend to install my theme in subdomain too if possible, or the image themes only.I know there are a lot of plug in to do this, but i am avoiding to use any plugin.
any help will be apreciatted.
thanks a lot.