That is the document I red. But I got confused. The following is what the document requires for preparing the Auxiliary Instance for TSPITR:

"Satisfy the following requirements before performing RMAN TSPITR:

* Create an Oracle Password File for the Auxiliary Instance

* Create a Parameter File for the Auxiliary Instance

* Start the Auxiliary Instance

* Ensure Net8 Connectivity to the Auxiliary Instance

* Start the Recovery Manager Command Line Interface "

I don't know if I need to create the Auxiliary database first.
Also I don't know how to use ORAPWD to create password file for Auxilary as there are more than one databases existing.