To get started with OMNI sources to build TWRP / MultiROM, 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/sub77/platform_manifest_twrp_omni.git -b twrp/mr-7.1
To initialize a shallow clone, which will save even more space, use a command like this:
repo init --depth=1 -u git://github.com/sub77/platform_manifest_twrp_omni.git -b twrp/mr-7.1
Then to sync up:
repo sync
Then to build:
cd <source-dir>; . build/envsetup.sh; lunch omni_<device>-eng; mka recoveryimage