Hi,
The directions for replacing the heater core are below. Before replacing the heater core, I would advise taking it in to have the evaporative emissions system leak tested with a smoke machine. Usually if you are getting antifreeze/coolant fumes you will either also have a leak showing at floorboard or when you turn on the defrost there will be coolant spraying on the inside of the windshield as a mist. An evaporative leak could cause gas fumes to get inside the car.
Heater core removal:
Automotive Repair Shop Manager
Automotive Repair Shop Manager, Technician