I used this tutorial video from OWC when I installed a SSD in my 27" 2011 iMac. If you go this route, just be gentle and take your time. Don't force anything, especially connectors.
I installed a Crucial M500 480GB SSD and left the existing 1 TB HDD. Works, excellent. The SSD was connected to the second SATA port on the Logic board and was installed under the optical drive.
I also installed 32GB of Crucial RAM purchased from Amazon.
You'll have a Whopping Fast iMac.