The dreaded 3 red rings of death for the Xbox 360.
I've done this repair on another Xbox without the Fixit kit, but I ordered the Fixit kit for this fix because of the nice tools and exact size bolts and washers that are needed to fasten the heat sinks.
I Followed the instructions (though they didn't come with the kit. Had to access them through the Fixit Website) and I turned it on. I still got the 3 red ring error. I then remembered from my first fix that I needed to turn on the xbox with the fan unplugged so the board and cpu and gpu would get hot enough to fuse the sinks. I think this is similar to what Fixit calls reflowing the board with a heat gun. After about 5 minutes the Xbox showed 2 red lights on the left (which I think means it shut itself off because of the overheating). I reconnected the fan and put the plastic piece on that directs the airflow to the CPU and turned on the Xbox again. Voila, the Xbox fired right up. I put it back together and I've been using it for about 4 days. I haven't had a problem yet.
You might need to "cook" the board after attaching the sinks by unplugging the fan and turning it on for about 5 minutes. The sink will get pretty hot. When the Xbox shows 2 red rings instead of 3 then turn it off and plug the fan back up with the plastic piece.
Disclaimer! Of course, I have to warn that this could possibly cause more harm than good if the board gets too hot. Do this suggestion at your own risk.