My late-2012 iMac 27" had slowed down to where boot up was taking 3-4 minutes! System continually locked up during the day.
I installed a Sony 850 Evo 500GB SSD and 8GB of additional memory. The instructions stated this was an advanced repair and not for beginners. I ordered parts from iFixIt, including the adhesive strips, which included the little wheeled separator for taking the computer apart. The kit comes with instructions, which I downloaded from this website and printed for reference. At first, the price seemed a bit high, BUT TRUST ME, once you've used this kit, you will agree that it is a deal, price-wise! I also bought the $56 tool-kit, which I used not only on this repair but also on two more repairs upgrading to SSDs on my other two computers. All three went without a problem and all three are running lightning fast with boot-times around 20-seconds and documents loads of milliseconds! I couldn't be happier with my purchases!
Everything was PERFECT! I was a bit intimidated by the process but quickly learned to trust the process.