Heater core

When you get the new core dunk it under water and blow air into one end while holding your thumb over the other and make sure there are no leaks (little air bubbles). You can always add more solder if you need to or if you feel like it is too thin in some places. I added more solder when I replaced the core in a 81 El Camino I used to have.
Other than that it is just as Longgone says in the other post, it's pretty easy.