If it doesn't work the first time, try and try again.
My Xbox 360 Elite (Original not Slim) had been Red Ringed for YEARS, and Destiny 2 was coming out soon and I hadn't played the expansions I bought for the first one yet. I also didn't even think to check out iFixit (even though I knew about them for years) until recently just out of curiosity. There were a few sets left at the time, so I figured to give it a go. I also wanted to potentially sell my 360 when Project Scorpio came out (or in general), even though it's only $20 bucks credit at GameStop.
The Repair was incredibly frustrating for me, but my case is unique. Quite literally, my case on my 360 was damaged from trying to open it prior without the appropriate tools, so with broken/missing clips, it made the removal of the case incredibly difficult (arguably the hardest part for me). I completed everything except the heat sink posts and put it back together. I didn't reflow the motherboard, nor attach the heat sinks the first time because I didn't have a heat gun, and the tool they provided to remove the bolts on the heatsink wasn't strong enough. After putting the console back together, I received another RRoD error code (one undefinable when searching online).
so I took it apart again, reattached the heatsink (again with the old posts), new thermal paste again, and put it all back together....Same issue. So, after a few days of frustratingly taking my console apart (breaking the case more and more everytime) I decided to finally invest and get an adjustable wrench, and a minature blowtorch (with a heat gun add-on). I reflowed the motherboard (of okay quality (messed up once, but no real damage)) and tried my hand at the heat sink posts again. The wrench was able to successfully remove the heatsink posts, and I was on my way....until...
The new heatsink posts somehow needed to put 8 washers (2x4) on top of the 4 screws which are inserted into the motherboard before placing the heatsink onto those screws. If that sounds confusing and/or difficult, you're right. I got so much thermal compound on me from trying to put that together, and failing multiple times. It could've gone easier if I had another person with me. But, after many trials and tribulations, I got both heatsinks on with the new posts. I put back together the console and angrily connected everything, saying "IF THIS DOESN'T WORK, I'm F*ING DONE"...and VOILA!!! IT WORKED! AFTER 3 YEARS! FINALLY WORKED! Got Network Settings, Xbox Live, and Destiny (with SO many updates) up and running almost no problem!
The provided heat sink posts are IMPORTANT. Every step they give you is important. Don't skip out on anything. If you need additional tools, then get them, because it will work. Even if some steps can be a lot more difficult than others (which isn't necessarily described nor shown in the manual.)