Creating read only apps schema in R12.1.3

Hi All, I am using (attached) steps to create read only apps user, but it seems the steps are for creating read only apps schema in R12.2. Can you please help me with the steps to create the same in R12.1.3. I am getting below error while executing:

SQL> alter session set current_schema=APPS;

Session altered.

SQL> exec AD_ZD_PREP.REGISTER_CUSTOM_USER('APPS_SELECT');
BEGIN AD_ZD_PREP.REGISTER_CUSTOM_USER('APPS_SELECT'); END;

  *

ERROR at line 1:
ORA-06550: line 1, column 7:
PLS-00201: identifier 'AD_ZD_PREP.REGISTER_CUSTOM_USER' must be declared
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored