How to enable flahsback on oracle database?

Steps to enable flahsback on oracle database:

connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, Real Application Clusters, OLAP, Advanced Analytics
and Real Application Testing options

set the recovery file dest location & size & set the flashback retention

SQL> alter system set db_recovery_file_dest='/media/sf_12cR1/flashback' scope=both;

System altered.

SQL> alter system set db_recovery_file_dest_size=100g scope=both;

System altered.

SQL> show parameter db_recovery_file_dest

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest                string      /media/sf_12cR1/flashback
db_recovery_file_dest_size           big integer 100G

SQL>  shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area 2137886720 bytes
Fixed Size                  2290416 bytes
Variable Size            1711279376 bytes
Database Buffers          419430400 bytes
Redo Buffers                4886528 bytes
Database mounted.
SQL>  alter database flashback on;

Database altered.


SQL> alter system set db_flashback_retention_target=6000;

System altered.

SQL>  SELECT NAME, FLASHBACK_ON FROM V$DATABASE;

NAME      FLASHBACK_ON
--------- ------------------
CDB1      YES


SQL> alter database open;


Database altered.