A: Create redo threads that have at least two redo logs for each additional instance.
B: If your single-instance database was using automatic undo management, then create an undo tablespace for each additional instance using the CREATE UNDO TABLESPACE SQL statement.
Note:
ABF: You can use Database Configuration Assistant to convert from single-instance Oracle databases to Oracle RAC. DBCA automates the configuration of the control file attributes, creates the undo tablespaces and the redo logs, and makes the initialization parameter file entries for cluster-enabled environments. It also configures Oracle Net Services, Oracle Clusterware resources, and the configuration for Oracle RAC database management for use by Oracle Enterprise Manager or the SRVCTL utility.
References: https://docs.oracle.com/cd/B28359_01/install.111/b28264/cvrt2rac.htm