Discharge A/C system.
Remove instrument panel. See: Body and Frame\Interior Moulding / Trim\Dashboard / Instrument Panel
Remove housing support bracket.
Disconnect heater core hoses and plug heater core outlets (to prevent coolant spills).
Remove heater box/evaporator housing.
Installation Install seals around perimeter of heater core so there are no gaps, and glue in place.
If heater core does not lock into place during installation, fasten to heater box using screws through tabs on top of heater core.
When replacing the heater core (1) with fresh/recirculating air flap, Install vacuum hose adaptor (2) on new heater core.
Replace gasket (3).
Remainder of installation is the reverse of removal.
as you can see, lots of labor intensive work.