DO make sure that you’re using the right guide. The code (EMC 2308) is embossed on the base of the iMac stand.
I used a USB to SATA cable (USB to SATA) to reinstall a working OS onto the new SSD and then backed it up from Time Machine. This meant that I knew I had a working replacement BEFORE I opened up the machine to replace the old disc with the new one.
There’s a lot of confusing information about the thermal sensor here, BUT if you’re replacing a conventional HDD with an SSD, you just need to make sure that you’ve got the appropriate SATA bridge with the sensor built in: Thermal Sensor NO fan problems!
I had no problems disconnecting and reconnecting the four main links behind the LCD screen and felt much happier having painless access into the main body of the computer, while the LCD sat safely on a clean cloth, well away from the dusty interior of the open iMac. After procrastinating for a year. This turned out to be painless!
Note - If you’re replacing the conventional HDD with an SSD, and using the appropriate thermal coupler, you only need to detach the sensor from the main board, you can leave the other end on the Hard Drive you’re removing. This also means that you have no need to worry about the orientation of the connector.
There are several alternative mounts out there. I used this one:
It worked excellently and seems less complicated than the one above.