| I have just started experimenting with this myself.
(Thanks to a post here;) I found that apt-move will take the packages from /var/cache/apt/archives and move them into a Debian package tree. Afterwards, it is simplicity itself to point a webserver at that tree and use it as a local archive. I don’t think I found a web page explaining how to do this exactly, but that is probably because the documentation explained it all.
One thing that caught me out, you have to generate a packages list on the server before you can start apt-getting.