Flip over the iMac.
Remove the screws from the bottom. You'll be able to lift the bottom plastic up from the round end of the iMac, then release the CD drive end.
Remove the 7 (i think) screws that hold the metal grating on, wiggle the grating free.
Disconnect the drive cables.
Remove the screws that hold the drive carrier in.
Pull out the drive carrier.
It should be self-explanatory from that point.
tooki