There's an excellent u-tube video on doing the ZJ heater core flush, by a fellow who calls himself, (somebody?) the car guy.
Just Google it.

 

Is this the video you are refering to?

 

Just unhook both hoses and flush both directions til it flows clear. You can unhook at the core, or at the engine end of the hose, whichever is more convenient.

I find the best flush comes from having the hose directly on the core, and using a jet nozzle to generate turbulence.

 

Yes, that's the one I was thinking about. If I remember right, he talks about putting "a product" in to the heater core, to help clean it.
I've heard some poeple use CLR, to remove any gunk, just rinse the core well, if you do use it.

 

Could I just use simple green?

 

I would be kinda hesitant about using simple green. I am not sure how corrosive that stuff is. Vinegar might work well for removing the crud, always works wonders in my coffee maker.

 

i've always used water and calgon. (the bath soap stuff)

 

Use flush chemicals with caution, if there's a corrosion problem you may end up with a leaker.