Hi,
aside from using git, is pretty much like using CVS, but taking into account that the non-core packages are in different repositories.
So, for instance, one could clone openacs-core in, say, /openacs/, and then go to /openacs/packages/ and clone each non-core package required individually in its own directory.
/openacs/ $ git clone https://github.com/openacs/openacs-core.git .
Cloning into '.'...
...
/openacs/ $ cd packages/
/openacs/packages $ git clone https://github.com/openacs/news.git
Cloning into 'news'...
...
The main issue is how to manage all the repositories in an orchestrated way afterwards, for upgrades, branch management and such. There are some tools available out there, like myrepos[1], to achieve that.
Also, for local development purposes, it might be a good idea to use local branches in a separate remote, and merge/rebase upstream's code into those.
For instance, having a 'github' remote, and an 'origin' remote with the local branches, and daily merge 'github/master' into 'origin/development'. This is also nice in terms of git history, as it makes easy to distinguish between local code and upstream's new code.
[1] https://myrepos.branchable.com/
Héctor