You rip them from disk to a computer storage device in an AppleTV compatible format using Handbrake or one of many similar computer apps - then import the resulting file(s) into your iTunes library.
I use Handbrake (free) which has been already mentioned and then run the encoded movie through [URL="https://itunes.apple.com/ca/app/iflicks-2-your-videos-in-itunes/id731062389?mt=12&ign-mpt=uo%3D4&at=10l3Vy]iFlicks 2[/URL] ($28.99) to fill in all the meta data. iFlicks 2 then imports it into my iTunes library which allows me to view it on my ATV through Home Sharing. Very simple to do.