To get started with OmniROM, you'll need to get familiar with Git and Repo.
To initialize your local repository using the OmniROM trees, use a command like this:
repo init -u https://github.com/ISTweak/android.git -b android-9.0
Then to sync up:
repo sync
Then to build:
cd <source-dir>; . build/envsetup.sh; lunch omni_hammerhead-eng; make recoveryimage