Start Standby Database

startup nomount
alter database mount standby database;
alter database recover managed standby database disconnect from session ;

Disable/Enable archive log destinations

alter system set log_archive_dest_state_2 = 'defer';
alter system set log_archive_dest_state_2 = 'enable';

To remove a delay from a standby

alter database recover managed standby database cancel;

alter database recover managed standby database nodelay disconnect;

Stop and Start of Logical standby apply

alter database stop logical standby apply;
alter database start logical standby apply;

Physical Standby switchover:

ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY;
SHUTDOWN IMMEDIATE;
STARTUP NOMOUNT;
ALTER DATABASE MOUNT STANDBY DATABASE;

In standby Database:

ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY;
SHUTDOWN IMMEDIATE;
STARTUP;

In Primary Database:

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION; 

If the primary Database is down,we can use fllowing step to active standby database:

Alter DATABASE RECOVER MANAGED STANDBY DATABASE FINISH;
Alter DATABASE COMMIT TO SWITCHOVER TO PRIMARY;
SHUTDOWN IMMEDIATE;
STARTUP;

Register missing archive log file
Find archive log gap by query:

SELECT THREAD#, LOW_SEQUENCE#, HIGH_SEQUENCE# FROM V$ARCHIVE_GAP;

ALTER DATABASE REGISTER PHYSICAL LOGFILE 'filespec1';