I have already installed the second SSD drive, but due to the size of the drive I need to change it with another SSHD 1 TB drive. Am I right that I have to follow the instructions up to Step 18? Because The SSD is already behind the optical drive, so what I need to do is remove it and install the new SSHD drive and put everything back again. Please confirm.

I have successfully installed a Samsung 840 Pro Series SSD 256 GB inside my Mid 2011 iMac following this great guideline. However, I am thinking of replacing the SSD with Seagate 1TB SSHD because of the capacity. I am not sure whether I need to put it in an enclosure or I can attach it as a bare drive? Please advise.

You are correct. I followed the ifixit guide on youtube and it was not mentioned to remove the RAMs. But in fact the RAMs should be removed in order to pull the logic board.