1)set ORACLE_HOME,ORACLE_SID
2)Create Database (run your create db scripts or use dbassist utility to create database)
3)Modify oratab entry to make the new instance active by changing "N" to "Y". You find oratab in /var/opt/oracle or search for it using 'find' command.
4)Make sure you have an entry in the listener.ora/tnsnames/names for new db
5)you are set to go from here.