Setup the build environment following the usual AOSP/LineageOS guide.
Setup your local sources using the LineageOS and athene sources as follows:
repo init -u git://github.com/LineageOS/android.git -b lineage-17.1
git clone https://github.com/rajatgupta1998/local_manifests.git -b lineage-17.1 .repo/local_manifests
Then to sync up:
repo sync --force-sync --no-tags --no-clone-bundle --optimized-fetch --prune -c -j$(nproc --all)
For building the final ROM zip:
. build/envsetup.sh && brunch lineage_<device>-userdebug -j$(nproc --all)
For any assistance or dev support, feel free to drop an email or a message at XDA or Telegram.