Hey guys, going to do a coolant flush on my 2000 Camaro this weekend. 3.8L Automagic. Coolant looks kinda nasty & I also have a stuck open thermostat (I think). It doesn't fully warm up in the winter unless I'm idling in traffic (probably gets to like 150 rather than 190 approx.). Other than that the car runs great, no loss in coolant or oil, but there's probably a fair chance there is rusty or corroded components inside. Bought the car in 2014 & my mechanic did a flush in 2020. Other that that I have no idea what the service history was on cooling system. Original radiator/ heater core. Water pump went in 2019.

Back to the coolant flush part, do you guys have any tips or recommendations? Never personally done this before. I looked up some youtube videos, just looking for any other input. 

Oh, and are there any additives that'll help clean better? I think I saw Chris Fix mention something in his video, I plan on revisiting that. 

Thanks for your help!