To get started with Android One, you'll need to get familiar with Repo and Version Control with Git.
To initialize your local repository using the New One Android trees, use a command like this:
repo init -u git://github.com/karon8/android.git -b cm-12.0
Then to sync up:
repo sync
All supported devices are built nightly and periodically as changes are committed to ensure the source trees remain buildable.