Set up your environment by referring to LineageOS Wiki (mainly "Install the build packages" and "Install the repo command").
Create a new working directory for your LineageOS build and navigate to it:
mkdir lineage-21-build-gsi; cd lineage-21-build-gsi
Initialize your LineageOS workspace:
repo init -u https://github.com/LineageOS/android.git -b lineage-21.0 --git-lfs
Clone both this and the patches repos:
git clone https://github.com/AndyCGYan/lineage_build_unified lineage_build_unified -b lineage-21-light
git clone https://github.com/AndyCGYan/lineage_patches_unified lineage_patches_unified -b lineage-21-light
Finally, start the build script - for example, to build for all supported archs:
bash lineage_build_unified/buildbot_unified.sh treble 64VN 64VS 64GN
Be sure to update the cloned repos from time to time!
This script is also used to make device-specific and/or personal builds. To do so, understand the script, and try the device and personal keywords.