So easy ... even a caveman can do it.
I did mine a while ago. There is only one screw on my iMac. If I recall correctly, it will stay put in the trap door. You may need something to put in one of the holes in what I call the trap door, so you can pull it loose from the computer. Removing and/or installing RAM was easier for me, than it was getting the cover off, then back on. Do make sure you are getting the correct RAM. I do believe there is a difference between desktop and laptop memory. A very cool friend sent me RAM that was lying around after he upgraded. So I did not have to worry. I do believe the iMac uses laptop memory modules.