Undo the 4 torx screws along the bottom and also remove the memory access door. Find a credit card size piece of plastic (ie bendable without breaking yet rigid). Apple calls it an "access card" but you can use an expired credit card. Bend one end to a 30 degree angle starting at about 1/3 of the way down the card. Slide the card up into the iMac through the vent slot and aim the card for the far corners of the iMac. You are trying to push a latch that holds the top corners of the plastic front frame on. While you are pushing the card onto the latch, start pulling the front bezel away from the frame. Repeat on the other side. With the iMac layed down screen up, flip the bezel over the top of the iMac so it will rest out of the way. Now remove all of the EMI shielding tape and peel back the EMI shield (the big black thing at the bottom). You can remove the LCD panel using a magnetic T10 driver. You want it to be magnetic because it is a PITA to get it back on with a non-magnetic driver. With the screen off, you have ready access to the hard drive. If you want to access the CPU, you will need to remove all the connectors to the logic board (be careful, some are quite thin and breakable) as well as the 11 or so torx screw holding it down. Be sure to mark where each screw came from as there are 5 types. With the logic board removed, you can remove the heatsink from the GPU and CPU giving you access to the CPU socket.
Have fun!