To get started with Android/Project-Diablo, you'll need to get familiar with Source Control Tools.
To initialize your local repository using the LineageOS trees, use a command like this:
repo init --depth=1 -u https://github.com/ProjectDiablo/android.git -b lineage-21
Then to sync up:
repo sync --current-branch --force-sync --no-clone-bundle --no-tags --optimized-fetch --prune -j$(nproc --all)
You can use your Lineage OS trees to build Project Diablo without any changes.
. build/envsetup.shlunch lineage_<devicecodename>-userdebugmka bacon