Taking an imac apart isn't easy although 2011 is better than the thin models. I installed an SSD in my 2010 iMac and it involved some swearing and I learned a lot if I ever do it again.
Best to read a guide and make sure you have the correct tools up front. Taking the screen off you have to be careful not to get it dirty or fingerprints. If you're not comfortable working on electronics I would suggest you find someone to to the work. If you've ever added parts to a desktop computer before you can do this but again read a guide or watch a YouTube video of someone opening up a 2011 iMac to see what is involved.