You should not attempt to replace a heater core in your Ford Focus without consulting an expert first. Replacing the core from the heater itself may be easy, but you must reach the heater from behind the instrument panel inside the car. And there are some models where a non-expert can't replace the core at all.
Drain the Focus' cooling system and air conditioner. Make sure you recover at least the air conditioner refrigerant. Disconnect the cable from the negative battery terminal.
Disassemble the dashboard and remove it with an assistant's help. This includes disconnecting the radio, steering wheel, glove compartment, ash tray, air bags, center console, defroster tubes and all trim panels, bolts and retaining screws.
Disconnect the hoses/lines to the heater, A/C evaporator and rear footwell vent along with the cross vehicle beam and heater housing wiring harness. Remove the heater housing by disconnecting all fasteners and remove the core from the housing.
Install the new heater core. Install the heater housing back into the engine compartment and connect the wiring harness, cross vehicle beam and all hoses/lines.
Reinstall the instrument panel (again, with an assistant's help) and reconnect all components, including the center console, air bags, glove compartment, steering wheel and radio.
Reconnect the negative battery cable. Refill the cooling system and recharge the air conditioning system. Start up the engine, and check for any leaks within the system.
The heater core cannot be removed from a 2005 Ford Focus except by an expert.