Level 0 - No trace. Just like switching sql_trace off.
Level 2 - The equivalent of regular sql_trace.
Level 4 - The same as level 2, but with the addition of bind variable values.
Level 8 - The same as level 2, but with the addition of wait events.
Level 12 - The same as level 2, but with the addition of both bind variable values and wait events.

SQL> ALTER SESSION SET EVENTS '10046 trace name context forever, level 12';
-- Execute the queries or operations to be traced here --
SQL> ALTER SESSION SET EVENTS '10046 trace name context off';

OR

SQL> EXEC DBMS_MONITOR.session_trace_enable; (OR)
SQL> EXEC DBMS_MONITOR.session_trace_enable(waits=>TRUE, binds=>FALSE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_MONITOR.session_trace_disable;

OR

SQL> EXEC DBMS_SESSION.SESSION_TRACE_ENABLE(waits => TRUE, binds => FALSE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_SESSION.SESSION_TRACE_DISABLE();

OR

SQL> EXEC DBMS_SESSION.set_sql_trace(sql_trace => TRUE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_SESSION.set_sql_trace(sql_trace => FALSE);

OR

SQL> ALTER SESSION SET sql_trace=TRUE;
-- Execute the queries or operations to be traced here --
SQL> ALTER SESSION SET sql_trace=FALSE;

OR

SQL> EXEC DBMS_SUPPORT.start_trace(waits=>TRUE, binds=>FALSE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_SUPPORT.stop_trace;

Note: Need to install before use 
SQL> connect / AS SYSDBA
SQL> @?\rdbms\admin\dbmssupp.sql
SQL> GRANT execute ON dbms_support TO schema_owner;
SQL> CREATE PUBLIC SYNONYM dbms_support FOR dbms_support; 

-- Enable 10046 Trace on other's session

SQL> EXEC DBMS_SYSTEM.set_ev(si=>123, se=>1234, ev=>10046, le=>12, nm=>' ');
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_SYSTEM.set_ev(si=>123, se=>1234, ev=>10046, le=>0, nm=>' ');

OR

SQL> EXEC DBMS_MONITOR.session_trace_enable(session_id=>1234, serial_num=>1234); (OR)
SQL> EXEC DBMS_MONITOR.session_trace_enable(session_id =>1234, serial_num=>1234, waits=>TRUE, binds=>FALSE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_MONITOR.session_trace_disable(session_id=>1234, serial_num=>1234);

OR

SQL> EXEC DBMS_SYSTEM.set_sql_trace_in_session(sid=>123, serial#=>1234, sql_trace=>TRUE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_SYSTEM.set_sql_trace_in_session(sid=>123, serial#=>1234, sql_trace=>FALSE);

OR

SQL> EXEC DBMS_SUPPORT.start_trace_in_session(sid=>123, serial=>1234, waits=>TRUE, binds=>FALSE);
-- Execute the queries or operations to be traced here --
SQL> EXEC DBMS_SUPPORT.stop_trace_in_session(sid=>123, serial=>1234);

OR

Tracing a process after it has started. Please find out PID, SPID using below query 

select p.PID,p.SPID,s.SID 
from v$process p,v$session s
where s.paddr = p.addr
and s.sid = '&SESSION_ID'
/

SPID is the operating system Process identifier (os pid)
PID is the Oracle Process identifier (ora pid) 

Using SPID (os pid)

connect / as sysdba
oradebug setospid 9834
oradebug unlimit
oradebug event 10046 trace name context forever,level 12
-- Execute the queries or operations to be traced here --
oradebug tracefile_name; -- display the current trace file.
oradebug event 10046 trace name context off

OR

using PID (ora pid)

connect / as sysdba
oradebug setorapid 9834
oradebug unlimit
oradebug event 10046 trace name context forever,level 12
-- Execute the queries or operations to be traced here --
oradebug tracefile_name;  -- display the current trace file.
oradebug event 10046 trace name context off

-- Level 12 Trace on service_name (waits=>true,binds=>true)

SQL> EXEC dbms_monitor.serv_mod_act_trace_enable (service_name=>'orcl',waits=>true,binds=>true);
	-- Execute the queries or operations to be traced here --
SQL> EXEC dbms_monitor.serv_mod_act_trace_disable (service_name=>'orcl');

-- Level 12 Trace on Module (waits=>true,binds=>true)

SQL> exec dbms_monitor.serv_mod_act_trace_enable(service_name=>'orcl', module_name=>'TOAD 9.5.0.31', waits => TRUE, binds => TRUE);
	-- Execute the queries or operations to be traced here --
SQL> exec dbms_monitor.serv_mod_act_trace_disable(service_name=>'orcl', module_name=>'TOAD 9.5.0.31');

For all sessions in the database

NOTE: Please be cautious when setting system wide, as this will impact performance due to every session being traced.
Warning: Setting the 10046 event for the entire database is not recommended. This can cause severe performance issues.

Existing sessions will not be traced. 
 
EXEC dbms_monitor.DATABASE_TRACE_ENABLE(TRUE,TRUE);
EXEC dbms_monitor.DATABASE_TRACE_DISABLE();

alter system set events '10046 trace name context forever,level 12';
alter system set events '10046 trace name context off';

Trace file named with spid under user_dump_dest location
-- To find SPID
select p.PID,p.SPID,s.SID 
from v$process p,v$session s
where s.paddr = p.addr
and s.sid = '&SESSION_ID'
/

eg: instancename_ora_spid.trc i.e orcl_ora_4393.trc

show parameter user_dump_dest
cd user_dump_dest location
ls -ltr *ora_spid*.trc
eg: ls -ltr *ora_4393*.trc

When using the DBMS_MONITOR.SERV_MOD_ACT_TRACE_ENABLE procedure, tracing
information is present in multiple trace files and you must use the trcsess tool to collect it into a single file.
The trcsess utility is useful for consolidating the tracing of a particular session or service for performance or debugging purposes.

trcsess [output=output_file_name]
[session=session_id]
[clientid=client_identifier]
[service=service_name]
[action=action_name]
[module=module_name]
[]

eg: 
cd user_dump_dest
trcsess output=one_big.trc service=orcl *.trc
ls -ltr one_big.trc

The tkprof utility parses SQL trace files to produce more readable output.
tkprof does not report COMMITs and ROLLBACKs that are recorded in the trace file.
Note: Set the TIMED_STATISTICS parameter to TRUE when tracing sessions because no time-based comparisons can be made without this. TRUE is the default value with Oracle Database 11g.

eg: 
tkprof one_big.trc output=one_big.txt sys=no

[oracle@rac1 trace]$ tkprof one_big.trc output=one_big.txt sys=no

TKPROF: Release 11.2.0.3.0 - Development on Sun Sep 11 16:50:56 2016

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.


[oracle@rac1 trace]$ ls -ltr one_big.txt