To get started with building Pixel Experience GSI, you'll need to get familiar with Git and Repo as well as How to build a GSI.
First, create a new working directory for your Pixel Experience build and navigate to it:
mkdir pixel; cd pixel
Clone this repo:
git clone https://github.com/ponces/treble_build_pe -b twelve
Finally, start the build script:
bash treble_build_pe/build.sh