You need to be logged in as a user with DBA privileges.

1) Check Current Archive Log Mode Status

Archive log list;

2) Shutdown the RAC database on each instance or simply shutdown from SRVCTL command:

srvctl stop database -d ORCL

Or 
- Need to run on each instance of RAC database. so use srvctl for shutdown
Shutdown immediate 

3) Start the Oracle RAC Database in Mount Mode

srvctl start database -d ORCL -o mount

OR

startup mount   ---start in mount state with any instance

4) Set the following parameter used for enable archive log:

SQLPLUS / as sysdba
ALTER SYSTEM SET log_archive_dest_1='location=/u01/archive/' SCOPE=spfile;
ALTER SYSTEM SET log_archive_format='arch_%t_%s_%r.arc' SCOPE=spfile;

5) Enable the archive log in mount state:

ALTER DATABASE ARCHIVELOG;

6) Open the Database

srvctl stop database -d MYDB
srvctl start database -d MYDB