pg_partman is an extension that simplifies the process of partition management in postgres.
Below are the steps that I followed to install the pg_partman extension with postgres 12.8.
Change your current working directory to the directory where you unzipped the postgres 12.8 source code.
git clone https://github.com/pgpartman/pg_partman.git
make PG_CONFIG=<pghome>/bin/pg_config NO_BGW=1
Then Edit your postgresql.conf file and add pg_partman_bgw to the parameter shared_preload_libraries
Now restart your postgres instance
At this time you are ready to create the extension from postgres and use it.
Use psql to login to your database
CREATE SCHEMA partman;
CREATE EXTENSION pg_partman SCHEMA partman;
\dx (To list the extensions and the version installed in your database)