I have not done it, but I have read everything on this and other forums regarding ssd installation on imac mid 2011 21.5 .
You need:
* ssd data cable, which is a standard sata data cable
* ssd power cable which is non standard. Part number is 922-9862 and the only place I found that sells it is
http://www.welovemacs.com/9229862.html .You can also try to make one yourself, see here
https://forums.macrumors.com/threads/1159154/ post #141
*Some velcro or adhesive tape to secure the ssd to the imac chassis.
The only place that has imac 2011 21.5 teardown instructions is ifixit
http://www.ifixit.com/Teardown/iMac-Intel-21-5-Inch-EMC-2428-Teardown/5485/1
You surely have to lift the logicboard a little, as the sata data and power ports are on the back side of the logicboard as you can see here
http://i419.photobucket.com/albums/pp280/dschlessman/6ce6eef9.png
I don't know if you have to remove the logicboard completely.
You have to put the ssd behind the optical drive. There is enough space there.
As I said before, I have not done it but I am pretty sure this is all the information that is available for mid 2011 21.5 ssd installation, all in one place.
If you do decide to do it, don't forget to document everything with fotos and video and post here your experience.
