Remove the front panel connector. (You may also need to remove an additional screw that connects the front panel connector to the logic board assembly).
The logic board assembly. This is the heart of the computer, it contains the motherboard, CPU, RAM, V-RAM, PRAM battery, the hard drive, and the optical drive.
To remove the bottom cover, remove these phillips screws, and pull the bottom cover off the iMac. (Start with the very back and use a flat head screwdriver to pop the main clip. The rest will come off relatively easily.)