PostgreSQL: How to enable autovacuum?

You can start the vacuum manually also. By running psql command vacuum full analyze verbose. It will take some time.

Autovacuum is on by default. For small databases just do nothing and everything will work fine. To confirm, check

SHOW autovacuum;

in psql. It should report on.

Large and busy databases sometimes require tuning to make autovacuum run more often, or focus more on busy queue tables. See the manual for details on tuning autovacuum.