1992 Chevy Corsica heater

Make sure it is your heater core, usually when the heater core goes, water pours out of the overflow drain line coming out of the car body fire wall.
Locate the heater water inlet and outlet lines, you may have to put car on lifts, or ramp stands to get to heater core.
Disconnect negative wire off the battery, drain coolant out of cooling system.
Disconnect heater hoses from heater core.
Insert coupling between inlet and outlet hoses and tighten clamps. Add coolant. Reconnect battery. Start car to test lines.