To start the instance:
# check status of instance
$ $CRS_HOME/bin/crs_stat

# seen cases when CRS says OFFLINE but pmon is still running so double checking here
$ ps -ef | grep <instance_name> | grep pmon

# start one of the instance on one of the nodes if not running
$ srvctl start database -d <database name> -i <instance name>

To shutdown the instance:

# verify instance running
$ ps -ef | grep <instance_name> | grep pmon

# shutdown one of the instance on one of the nodes
$ srvctl stop database -d <database name> -i <instance name>

# check status of instance to make sure it’s down
$ $CRS_HOME/bin/crs_stat

# verify instance not running
$ ps -ef | grep <instance_name> | grep pmon