1) Find a database on a server separate from the target database. If you don't have such a database, you will have to create one.
2) Create a user in this database and make sure it can create tables in a tablespace in that database. You can call this user anything you want. In my environment, I made things easy and called this user 'rman'.
3) Grant the RECOVERY_CATALOG_OWNER role to this user.
4) Fire up RMAN at the command prompt.
5) Issue the command to connect to the target database.
6) Issue the command to connect to the catalog database using the user you created in step #2.
7) Issue "CREATE CATALOG;" to create the Recovery Catalog in the catalog database.
All of these steps are fully documented in the Oracle RMAN documentation. I highly recommend reading it as RMAN is a little complex and takes a while getting used to it.