How to replace the heater core step by step?

You will need to drain the coolant and disconnect the battery, it shows you may be able to sneak the heater core out of the HVAC plenum without the vacuum down of the A/C system. If not, the dash will need to be removed so the HAVC plenum can come out of the car in which case, you will need to do the A/C service. Here are the steps it will take to replace the heater core. Let me know how to goes, and upload images if you get stuck so I can help.

Here is a video to help you vacuum down and recharge the system:

https://www.youtube.com/watch?v=4EqdrBVb0sY

Check out the images (below).