Firstly, you need to access the iMac's hinge. Find the appropriate disassembly guide for your iMac. I used iFixit's guide for the logic board. You don't need to follow it exactly.

Speakers don't need to be removed. You can simply loosen them and carefully move them out of the way, in order to remove the necessary components.

The hard drive, logic board, power supply and fan need to be removed.