How to generate sql explain plan?

e.g.
prepare example table:
SQL>
create table ep_employees as select * from employees

observe costs values:
SQL>
explain plan for select * from ep_employees where job_id = 'SA_MAN';
-- Explained.

SQL> 
select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
-------------------------
Plan hash value: 745119821

----------------------------------------------------------------------------------
| Id  | Operation         | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |              |     5 |   675 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EP_EMPLOYEES |     5 |   675 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("JOB_ID"='SA_MAN')

Note
-----

   - dynamic sampling used for this statement (level=2)

Fix full access issue on ep_employees table:

SQL>
create index ep_emp_job_ix on ep_employees (job_id);

SQL> 
select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------
-------------------------
Plan hash value: 3994861717

---------------------------------------------------------------------------------------------
| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |               |     5 |   675 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EP_EMPLOYEES  |     5 |   675 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EP_EMP_JOB_IX |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("JOB_ID"='SA_MAN')

Note
-----

   - dynamic sampling used for this statement (level=2)

Results:
Costs are reduced so sql query performance got increased.

How to enable 'Diagnostic' and 'Tuning' diagnostic packs?

Check parameter
SQL> SHOW PARAMETER control_management_pack_access

Solid solution
add this into initora file;
*.control_management_pack_access='DIAGNOSTIC+TUNING'  

Revoke the operation temporary
SQL> ALTER SYSTEM SET control_management_pack_access=NONE;