All credits goes to OMNI contributors and TWRP contributors!
To get started with OMNI ROM, you'll need to get familiar with Git and Repo.
To initialize your local repository using the OMNIROM trees to build TWRP, use a command like this:
repo init -u git://github.com/vineethrp/platform_manifest_twrp_omni.git -b twrp-7.1
To initialize a shallow clone, whcih will save even more space, use a command like this:
repo init --depth=1 -u git://github.com/vineethrp/platform_manifest_twrp_omni.git -b twrp-7.1
Then to sync up:
repo sync
Then to build:
cd <source-dir>; . build/envsetup.sh; lunch omni_g15v97_gq-userdebug; mka recoveryimage