In addition to what Robert said, there are 4 or 5 bolts under the hood on the firewall that need taken off. There is a bolt on top of the heater box inside the car that needs undone, the vacuum hoses, a couple of electrical connectors, the cable from the heater/ac control panel. It is a lot easier if the front seat has been removed from the car.

Under the hood are the heater hoses and the a/c hoses if you have a/c. If you have a/c and it was working, you may want a shop to vacuum out the refrigerant.

When I do the heater cores, I replace the A/C evap too. It is such a chore to take out the box, the extra $350 for an evap core is worth it.