Adding an Extra SSD to my iMac, guide?

Hey!
I´m intressted in putting an extra SSD drive in my iMac, i have a 1 TD right now and want to and an 240gb SSD to it, but how?
Is this even possible? iv heard many people talking about replacing the Original HDD but i don't want to replace it but add a new one. Iv also heard a lot of problems about the fans going way to fast when u add an sad, is this still an issue or how does on solve it? i also don't wish to remove my optical drive or anything like that, i just want to add.
I have the 2011, 27" model (bought in december 2011)
Does anybody have a guide or a how to video that can help?