The documentation is rather clear with examples like-
CREATE TABLE measurement_y2006m02 PARTITION OF measurement
FOR VALUES FROM ('2006-02-01') TO ('2006-03-01'); CREATE TABLE measurement_y2006m03 PARTITION OF measurement FOR VALUES FROM ('2006-03-01') TO ('2006-04-01');
Note- Don't partition on function results like date_part because performance will likely suffer greatly. Also note that the top end is always exclusive so the above give a continuous range for those two months.
I would hesitate to partition by more than year alone before upgrading to PG v12. The speed improvements for more than 10-100 partitions (max recommended for PG11) is huge in 12.