To get started with OMNI sources to build TWRP, 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 https://github.com/ONE-WearOS/manifest_twrp_onewear.git -b twrp-9.0
To initialize a shallow clone, which will save even more space, use a command like this:
repo init --depth=1 -u https://github.com/ONE-WearOS/manifest_twrp_onewear.git -b twrp-9.0
Then to sync up:
repo sync
Before building, make sure you have your system's python implementation set to python2. This project was created before python2 was deprecated and will not build with python3.
Then to build for a device with recovery partition:
cd <source-dir>; export ALLOW_MISSING_DEPENDENCIES=true; . build/envsetup.sh; lunch omni_<device>-eng; mka recoveryimage
Then to build for a device without recovery partition:
cd <source-dir>; export ALLOW_MISSING_DEPENDENCIES=true; . build/envsetup.sh; lunch omni_<device>-eng; mka bootimage