To get started with OmniDragon, you'll need to get familiar with Git and Repo.
To initialize your local repository using the OmniDragon trees, use a command like this:
repo init -u https://gitlab.com/holydragonproject/android.git -b hdo
Then to sync up:
repo sync
Then to build:
cd <source-dir>; . build/envsetup.sh; brunch <device_name>
Good luck!