Use OEM or
Use DBCA or
use the command line utility ‘rconfig’

Refer to the Oracle RAC  Installation guide for the complete instructions.

Some notes:
To use rconfig:
Ensure Oracle clusterware and Oracle database software is installed on all nodes in the cluster.
Ensure Oracle clusterware is started.
Ensure the oracle database binary is enabled for Oracfl RAC on all target nodes.
Shared storages is set up
User equivalency, SSH, is enabled.
the DB has been backed up.

Edit either:
$ORACLE_HOME/assistants/rconfig/sampleXMLs/ConvertToRAC_AdminManaged.xml or

Run rconfig, supplying the xml file name to use.

The buffer cache and shared pool capacity requirements in Oracle RAC are slightly
greater than the requirements for single-instance Oracle databases. Therefore, you
should increase the size of the buffer cache by about 10%, and the size of the shared
pool by about 15%.