RAC
sqlplus / as sysdba
SQL> alter system set cluster_database=false scope=spfile; --in node1
SQL> create pfile='/opt/oracle/pfile_20120301' from spfile; --in node1
SQL> alter system set log_archive_format='ora11g_%r_%t_%s.log' scope=spfile; --in node1, using default value is ok
SQL> alter system set log_archive_start=TRUE scope=spfile; --in node1
SQL> alter system set log_archive_dest='' scope=spfile; --in node1
SQL> alter system set log_archive_dest_1='LOCATION=/oracle/ora_arch' scope=spfile; --in node1 using the specificated DIR
SQL> shutdown immediate; --two nodes
SQL> startup mount; --in node1
SQL> alter database archivelog; --in node1
SQL> alter database open; --in node1
SQL> alter system set cluster_database=true scope=spfile; --in node1
SQL> shutdown immediate; --in node1
SQL> startup; --in two nodes

NORAC
sqlplus / as sysdba
SQL> create pfile='/opt/oracle/pfile_20120301' from spfile;
SQL> alter system set log_archive_format='ora11g_%r_%t_%s.log' scope=spfile; --using default value is ok
SQL> alter system set log_archive_start=TRUE scope=spfile;
SQL> alter system set log_archive_dest='' scope=spfile;
SQL> alter system set log_archive_dest_1='LOCATION=/oracle/ora_arch' scope=spfile; --using the specificated DIR
SQL> shutdown immediate;
SQL> startup mount;
SQL> alter database archivelog;
SQL> alter database open;
SQL> archive log list;