How do I replace the heater core? What is the easiest way?

What is your question?

I replaced my radiator last year and back flushed the heater core and still cold air comes out. I know the heater core is bad. I bought a new one and would like to know the fastest and easiest way to replace it before winter. I had to replace the radiator because the head gasket was blown and I added "stop leak" to the radiator. I notice the air was getting cool and the radiator was getting hot.