Could someone please direct me to a GOOD article on SQL Developer's Explain plan.
Do you mean the Oracle Database explain and execution plans? How about this article written by the optimizer group.
There is literally nothing I could find anywhere that really explains HOW to use it to tune a SQL statement. Being new to the tuning environment, I need something that goes step by step in explaining how to interpret the plan as a guide to modifying the SQL statements.
Performance Tuning is a vast subject and Oracle has written entire documents about it. This Database Performance Tuning Guide should keep you busy until the new year.