This will print the path

psql -c "SHOW config_file;"

1. Add it to the conf

   shared_preload_libraries = 'pg_stat_statements'	# (change requires restart)
   

2. Enable it using psql for the database of choice.

   psql -d mydatabase -c "create extension pg_stat_statements;"
   

Set the path using the PGDATA env var or set the data_directory variable in the conf.

Might consider checking this into source control for a given application.