Mid-2010 with stock iMac WD Caviar Blue 500 GB HDD failed after 9 years. I replaced it with a 1TB SSD using nothing more than Electrical Tape and a 15 cent transistor.
Reading the full set of instructions first was useful in discovering that the LCD panel has short cable lengths - A flashlight, and a plastic knife were majorly useful tools. The knife helped with gently removing or pushing the pins as required.
The HDD was easily removed. I kept the metal bar which held the original HDD and used Electrical Tape to mechanically fasten the SSD to the bar.
Using the 2N3904 transistor I pushed the Emitter pin into the grey wire pin of the thermal sensor cable and both Base + Collector pins into the black wire pin. (Original solution by Alexandre on iFixit).
I tested the setup by powering up the computer connected to an external display. Everything worked as expected.
Closed the computer back up, cleaned the LCD with crumpled newspaper (it works really well) and booted from a secondary SSD in the DVD slot.