To get started with this custom TWRP manifest, 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/TWRP-Mi-A2-Lite/platform_manifest_twrp_omni.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 git://github.com/TWRP-Mi-A2-Lite/platform_manifest_twrp_omni.git -b twrp-9.0
Then to sync up:
repo sync
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