I wanted to upgrade the sluggish 2.5" 5400 RPM platter drive with a new Samsung EVO 850 SSD, While I was in there, I figured I'd double the 8GB of RAM to 16GB.
Success! This guide is spot-on! I was able to follow each step in detail and safely open my iMac without any issues. The repair kit from iFixit was a godsend. The mini pizza cutter tool made quick work of the factory adhesive tape, and the included double-sided tape was a perfect match with simple templates to follow. The original drive came out very easily, and I was even able to painlessly upgrade the RAM. Before sealing it all up, I plugged in the power, confirmed it powered up, and the screen worked as well. All good!
It's a bit annoying having to carefully remove the factory adhesive tape, but go slow, use a plastic tool (like a spudger) to peel the tape off and once it starts to peel off, the it comes off quickly. The slowest part of the whole operation was removing the old tape. Also, I followed the advice of another person here who suggested coaxing the RAM out carefully without the need to fully remove the system board. I found plenty of room to remove the old memory and gently angle in the new; took me about 5 minutes total without the need to fully remove all the components. Good luck!